How to Fix Green Aquarium Water


Algae Green Aquarium Water

An algae bloom is the cause of the green aquarium water.  Basically, we have to figure out why too much algae is growing in your fish tank.  We’ll dive into the causes of aggressive algae growth and how to stop it for good.

Are You Overfeeding?

If you have food left over in your fish tank a few minutes after feeding you might overfeeding you fish.  If this is part of the problem there is a pretty easy solution.  Cut back on the amount of food you are feeding.  Just make sure everything is eaten at each feeding.

Are You Doing Frequent Water Changes?

A dirty tank will quickly grow algae.  Frequent aquarium water changes need to be done to keep algae under control.  When nitrates levels get high, algae thrives.  If you haven’t been doing water changes, you should start right away.  You should also do some pretty large (25%-30%), regular water changes to get your green cloudy aquarium water under control fast.

Is Your Tank Getting Too Much Light?

Algae also thrives on light.  Reduce the light and stop the algae growth.  If you tank gets direct sunlight you should find a way to limit the light your fish tank sees.  You might be able to just keep the blinds closed.  If not, you should consider moving your tank if your green aquarium water is a big problem.  You could also limit the amount of time your aquarium light is on.  This can contribute to algae growth as well.  Another short term option is to cover your tank with a blanket if direct sunlight is a problem.  This really isn’t a great long term solution as you fish and plants need light as well.  Once you get the algae problem under control, a regular maintenance schedule should be enough to keep things stable.

Chemicals as a Last Resort

There are chemicals that you can find at your aquarium retailer that will quickly kill off algae.  The only reason I would shy away from them initially is so you can figure out the cause of the problem.  If you do not find the cause, it won’t be long before you’ll see it show up again.  I will say, if it gets too out of control, it’s not a bad option for a quick temporary fix.  They usually work swiftly.

We’d love to hear about your green water!

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94 Responses to How to Fix Green Aquarium Water

  1. Lynn October 3, 2007 at 1:13 am #

    The water in my 5 gallon aquarium has been green for 2 months. I have been changing 25% of the water every day using water that has been bottled for two days and conditioner. I have changed the filter at least 4 times. I have kept the light off most of the time and it is not near a window. If I let the aquarium go for 2 days, the water is so green and murky that you can hardly see the fish. There is no algae visible in the tank other than the green water. This is a tropical fish tank with a heater. The temperature is always in the appropriate range on the thermometer. There are only about 7 small fish in the tank. I have one large algae eater (about 4″ long) that does not seem to be cleaning the tank. I just can’t think of anything else to do. Your help would be appreciated.

  2. Eric October 3, 2007 at 10:04 pm #

    Hi Lynn,

    Sound like you have it bad. Have you tried chemicals yet? It looks like you have done everything else outside of chemicals that you could have. The problem is that with your filter changes and aggressive water changes you may be doing more harm than good. Water changes that aggressively can put your bacteria filtration in jeopardy. Chemicals have side effects, but the benefits in this case should outweigh the cost.

    I would try a product like Algae Fix. Any product with the active ingredient of polyethylene dichloride should do the trick. Following that up with the attention you have already shown your tank should keep the problem in check.

  3. Rob October 24, 2007 at 2:46 pm #

    I also have green cloudy water. Its smelly as well. My tank gets very little natural sunlight, and only 9 hours artificial light. I’ve tried partial filter changes, water changes every 5 to 6 days and as a last resort have taken plants out of the aqaurium. The water is a little better after water changes but is still cloudy. After a few days its starts to turn green again. I only feed once a day and then only small amounts.
    Any ideas?

  4. Eric October 24, 2007 at 9:42 pm #

    Hi there Rob,

    I suggest stepping up the water changes to 20% once a day until you feel it has completely cleared up. The water changes you are doing might solve the problem for many, but it sounds like you’ll need to get a little more aggressive. You can gradually reduce the frequency when you are rid of the algae completely.

    If after a week or 2 of that regimen you are not rid of it, you should probably turn to a chemicals algae reducer.

  5. Crys October 31, 2007 at 9:52 pm #

    I have a 30 gallon tank and my water is green with an algae bloom. I have found that I have been giving my tank too much light. I have a snail inside my tank and I’ve heard that algae reducing chemicals may kill the snail. Are there any products that will be safe to use without harming the fish and the snail?

  6. Robbie November 1, 2007 at 1:11 am #

    I have a 55 gallon tank with 2 oscars that has been running for 5 months. It was ok for almost 2 months and then got green. I moved it out of direct light and changed all the water. I got some water from an established tank. It was fine for another month or so and now it is starting again. What should I do? I have tried water changes and some algae destroyer.

  7. Eric November 2, 2007 at 7:23 am #


    For your situation, I would recommend the water change regimen right above your comment. That should do the trick.

  8. Eric November 2, 2007 at 7:29 am #

    Hello Robbie,

    Overfeeding could be a cause. I would pay particular attention to that. Also, regular water changes seem like the key. I see you did a full change, but keeping up on them once a week is probably the key in your situation.

  9. Robbie November 2, 2007 at 12:35 pm #

    I have been changing water regularly. It seems like it is working for a day or so but then gets cloudy again. I have and aquaclear hanging filter for up to 90 gallons. Should I get a small fluval to help move more water?

  10. Eric November 6, 2007 at 10:22 pm #

    Robbie, I don’t think the Fluval will solve your problem. More than that, they aren’t cheap. I think the next step would be to completely block all light to your tank for 3-5 days with a cover or blanket Do this after a good 25% water change and you should see results.

  11. Heather November 8, 2007 at 2:24 pm #

    I have a 10 gal tank that has had incredible algea growth. I have been cleaning the tank regularly. First I changed 20-30% but I have had to increase the amounts because the algea overwhelms the fish. I have changed all the water, rinsed the rocks and everything. I know that this is not good for hte cycling but the a;gea has gotten so thick that the fish huddle together by the air because it is the only clear area. Please help. This problem has been going on for weeks. The water only stays clear for a couple of days before it gets completely filled with the stringy algea.

  12. Eric November 9, 2007 at 11:41 pm #

    Heather, Algae grows only if there are nutrients in the water and there is a light source. Without these it cannot reproduce. With the state of your tank, you’ll have to take extreme measures. Start with a large water change that clears the algae. You must be cleaning the waste off the bottom of your tank with a siphon when doing these water changes. You cannot just dump the water out. The key is to clean up the waste. Cut off light completely with a thick blanket or cover of some sort and cut back feeding to minimal levels. A minute or two tops, and your food should be eaten during this. After a few days of absolutely no light you should see serious improvement.

  13. Greg November 10, 2007 at 11:05 am #

    I have two freshwater tanks, a 55 gal and a 10 gal. Both tanks have very green water. I never used to have this problem in either tank and they ironically started turning green about the same time. One thing that occurred to me that around the time the green water started, our town water had been changed chemically. I believe they discontinued the use of chlorine and replaced it with something else (I can’t remember what). Could this be connected?

  14. Eric November 10, 2007 at 2:43 pm #

    Hi Gregg, I wish I had the answer to your question. I guess it would depend on what replaced the chlorine. It is important to note that water conditioner should help nutralize the effects of most tap water additives.

  15. Gazali November 10, 2007 at 11:30 pm #

    Green water or algae bloom usually means there is excess nutrients in the tank.

    I do not advocate using chemicals to destroy it simply because after the medication your beneficial bacteria in the filter can be wiped out. It will result in another cycling of the tank. I hate to go through that. Results: dead fishes.

    What I would suggest is this:
    Use a UV Sterilizer.

    Do a water change of about 50 to 60%.
    Fix up the UV Sterilizer.
    Let it run. You may need to change the water a couple of times to clear up the dead algae.
    Reduce feeding to once a day or once every two days. Don’t worry your fishes won’t starve to death. It should be clear in a couple of days.

    The other thing is check if your tank is in a direct sunlight position. It encourages algae growth. If you have lights then get a timer switch.

    There are a lot of brands of UV Sterilizer. Check with your local fish store. The sizing is usually dependent on the tank size. Generally they fall into two categories: Internal and external.

    The internal ones is easier to set up. Just put it in the tank and switch it on. The external ones require extra plumbing and a pump if it is not attached inline with a canister filter output.

    Of course you need to do regular maintenance on it and the UV bulb need replacement based on the manufacturer recommendations. So get a unit that is easily maintained and the bulb easily replaced.

  16. Sara November 17, 2007 at 10:13 pm #

    We have a 50 gallon tank with 3 gold fish and two sucker fish. We have had the tank set up for years and have never had a problem with the water clarity. About two weeks ago we did our regular monthly water change of somewhere between 30-40%. A few days later the water was very cloudy. Not knowing that I might have been making the problem worse, I preceeded to do small water changes (about 15%) daily. At this time I also changed all of the filters, including the wheel. It didn’t get better it actually got worse. The local pet store told me that it was most likely a bacteria bloom. They tested the water and it was fine. They said to sit tight and put some extra charcoal in the tank. This was three days ago. The water is now very green (it had just been cloudy/white) and so green that you have trouble seeing the fish. Can you help??

  17. Eric November 18, 2007 at 11:21 pm #

    Hi Sara, The simple steps are to cut back feeding to once a day, keep light away from the tank and wait a few days. If this doesn’t do the trick you really have two other choices. Chemicals that help remove the nutrients from the tank (they have their drawbacks, see above) or the UV sterilizer Gazali mentions above (great advice) The only reason I don’t normally recommend them is that they are not cheap. Most don’t want to spend the $100 or so. It is however, very likely that it a UV sterilizer would solve your problem.

  18. Sara November 18, 2007 at 11:30 pm #

    Thank you.

  19. Evan November 20, 2007 at 10:57 pm #

    Hi, I have plants, 7 fish, and a african dwarf frog. My tank is 10 gallons and looks like a swamp. In 2 weeks it has gotten bad. I scrub like every 2 days but just gets worse. I keep the blinds closed and only have the light on for 4 hours. Can you suggest alage eating animals and tips?

  20. Lynn November 25, 2007 at 1:17 pm #

    Hi, I started these questions circulating about not being able to cure my green tank water about a month ago. I tried all of the suggestions that were made and finally found a cure! I went about 2 weeks without doing anything to the tank except feeding the fish. (Did not help) and then I covered the tank to block all light for 3 days. I took off the cover and let it sit for two days (water much better but still somewhat green). I then changed the filter (very dirty-green stuff), within two days the tank was completely clear! It seems to have stabilized now and looks great! Thank you for all of your help.

  21. Cathy November 26, 2007 at 10:52 am #

    Hi, My daughter is doing a science fair project on th e green cloudy water in our tank – we have set up 3 small tanks. Any suggestions on how to set up the experiment?

  22. Eric November 26, 2007 at 9:23 pm #

    The Bushy Nose Plecostomus is know as one of the best algae eating fish around.

  23. Eric November 26, 2007 at 9:29 pm #

    Hello Lynn,
    Thank you so much for telling your story. It’s great to have stories contributed about what has worked and for that matter what hasn’t for everyone. Every story is a little different making for a great knowledge base. Thanks again!

  24. Eric November 26, 2007 at 9:34 pm #

    Hi Cathy,
    I guess it depends on what you are trying to show. You could set up a tank with extreme sunlight and dump in a ton of food to create more algae. Maybe one with clear water and no changes. Maybe another with algae treated with chemicals.

    Anyone else have any ideas?

  25. Cathy November 26, 2007 at 11:13 pm #

    Hi, Eric

    Thanks for your suggestions. We set up 3 small 1 gallon tanks and filled them up with the green cloudy water from our 29 gallon tank – I think we want to try to clear the water. We thought we might try covering one tank so it gets no light and see if that causes the water to clear up. The other tank would continue to get sunlight. Maybe the 3rd would be a control. We are trying to clear our big tank using a product called Algone. It is supposed to be a natural remedy for treating green cloudy water- so far no luck the tank is still just as green and cloudy as ever!! But I’m told we have to be patient and let this stuff work. I would appreciate any thoughts or comments.

  26. Cathy November 26, 2007 at 11:17 pm #

    Hi Eric,

    I really like your idea about having one tank that is clear and one that we continue to really feed the algae. Thanks!

  27. Eric November 28, 2007 at 6:39 am #

    Cathy, Sounds like a great experiment! I’d love to know how it goes!

  28. Lisa December 9, 2007 at 11:03 am #

    I hav ea very cloudy tank also. Its a 55 gal with 4 gwalimonie (spelled wrong), and 1 3-4 long algae eater. My water temp stays at 79, it’s about 5 feet from my sunny window with blinds down, and its 1 feet from my heater one of those stainless stell heaters that is about 6 feet tall and3 feet wide. I have change water every week and cleaned all fake plants well. Still cloudy. I can not move tank myself from window and was thinking to get black blinds. But any others suggestions you may have Eric.

  29. Eric December 11, 2007 at 7:04 am #

    Hi Lisa, With the blinds closed, I don’t think the window is your problem. It could be overfeeding. If you tank is more of a white cloudy, you should check this out article.
    If it’s green and you have had good maintenance/water changes you might consider a UV sterilizer or chemicals. Check out the discussion above for more info on UV sterilizers.

  30. Tyler December 13, 2007 at 11:26 pm #


    I have a 12 gallon Nano cube, we were going to do salt water, but decided to do fresh, and are thinking going to do salt water, anyway, it was fine for about 2 months, then everything went bad, not only was my tank almost “gatorade green” most of my fish, even danios, died!!!

    I have tried chemicals, they only worked for a week or so, water changes, even a 100% water change the other day! It is now not as bad, but still hard to see the fish only two left, and you cant see through the tank.

    I will try no-light trick but is there anything else i can do?

    Thank you

  31. Eric December 17, 2007 at 10:25 pm #

    Hello Tyler,
    Keep the aggressive water changes up (around 50% per day), no light, and cut back feeding to half of what you normally do. With this, it should clear up with a little time.

    Also, you might consider a UV sterilizer. Take a look at Gazali’s comments above. It’s a great way to treat algae if you have it bad.

  32. Eric January 3, 2008 at 9:35 pm #

    Here’s an email product suggestion from Tyler.
    Name of algae stuff us Algaefix, company name is API. This stuff cleared my tank up in 2-6 hours. My tank was cloudy green, & now it is crystal clear.
    Never used it, but looks like it did wonders for his tank. Thanks Tyler!

  33. Mike January 16, 2008 at 11:25 pm #

    I would like to know what is the cause of spotty algae growth on the sides of my 25 gallon aquarium. I have done partial water changes, however, my water is not a cloudy green. I have a Chinese Algae Eater, 2 Golden Snails, a plecostomus, an Upside Down Catfish, 4 Neon Tetras, a Dragonfish Eel, and a Red Tail Black Shark in the aquarium. I do not have a heater in it since the temp runs about 72-74 degrees constantly, and the water is filtered constantly. I changed the filter just recently. What do I need to do aside from using chemicals?? I want to use chemicals as a last resort.

  34. Eric January 19, 2008 at 2:07 pm #

    Hi Mike,
    Sound like brown algae?? This is typically caused by low light, excess nutrients (usually caused by over feeding), or infrequent water changes. It’s also more prevalent in newly set up tanks. In newer tanks the good bacteria is not fully established and cannot keep the above problem in check. My advice is to try to solve the problems above if one or more sounds like your case. One of these things is probably the root of your problem. Until you get it cleared up, you just need to get yourself a scraper or algae magnet. I shouldn’t take more thank a couple minutes to clean a 25 gallon tank.

  35. Andrea January 22, 2008 at 12:28 am #

    Hi Eric,
    When doing a water change, should I dig the siphon deep down into the gravel and move the gravel around (so as to get down to the glass bottom) or should I just skim the top of the gravel?

  36. Billy January 23, 2008 at 9:28 pm #


    I just happened to read some of your responses regarding the aquarium turned green over night/days without the appearance of algea. Your responsed were to change the water or cut feed would take care the problem. I hate to say it, but you were incorrected. Indeed, the real problem was the excess of vitamins B12 and a bad light bulb. Test your vitamin B12 level and replacing a bulb will take care the problem.


  37. Eric January 24, 2008 at 6:51 am #

    Andrea, You’ll want to dig into your gravel and get down to the bottom. That’s where you really see the work the siphon is doing. You’ll see all sorts of gunk that you are getting out of there.

  38. Eric January 24, 2008 at 7:13 am #

    Hey Billy, Thanks for your input! I want to make sure nobody is confused. A bad light bulb might the root of some of these problems. However, the likelihood of that being the cause of a number of these problems seems slim. It would be wise to check your bulb if you have a bacteria bloom. You’ll still need to quickly clean up that water. That is the reason for the dramatic water changes mentioned above. Over feeding is also a common occurrence that can lead to a bacteria bloom. This just seems like a more likely cause than a bad bulb for the many.

  39. steve January 27, 2008 at 9:00 am #

    Hey everybody Ive had the same problem with green water in my tank for about 6 months i did everything I could water changes every other day, filter changes even ran a diaton filter through my tank several times and none of that worked. I even went as far as setting up another tank thinking that would fix it I WAS WRONG. I finally fixed the problem over night litteraly. I had a heater in my tank that was heating the water up to much killing my fish ant turning my tank green. So my advice to you is if you have a heater in your tank unpluge it for a night and see if that is it. It worked in my case my tank went from green to crystal clear overnight.

  40. Steff and Mag January 27, 2008 at 10:48 am #

    I had a very bad experience of keeping fish. I got a new fish tank with some fishes shrimps and water plant. Firstly, my shrimp died because of the water that is not settle down. then my fishes contacted with ick and they all died. So I decided to keep just Red Crystal Shrimps. Everything was fine until 1week ago, the water suddenly turns cloudy and I have change the whole entire water but still the same. Im so sad, eventually all I want is to keep shrimps as a pet and every day this tank is giving me problem that i have to hunt into and spent onto.

  41. Eric January 28, 2008 at 10:35 pm #

    Hi Steff and Mag,
    Sorry to hear about your trouble. Don’t worry, I’m sure you can get it under control. I would suggest reading through the above comments. Be careful with complete water change outs. This will usually only encourage the bloom. You destroy your bio-filter and can really kick your bloom in to high gear. I would suggest some bacteria starter to kick things off again.

  42. Eric January 28, 2008 at 10:39 pm #

    Thanks for the input Steve! How high was the temperature at its worst?

  43. Steff and Mag January 29, 2008 at 11:01 am #

    Thank you Eric for your generous reply.
    Today I have another 2 shrimps died again and the fact is that I have already change the entire tank of water. so what am I suppose to do to stop my shrimps from dying?

  44. Eric January 29, 2008 at 9:35 pm #

    Hey Steff and Mag, No problem. Do you have a test kit? After testing, what do you see that’s out of whack? You should quickly get the bacteria starter in there.

    Also take look here to learn more about regular water changes:

  45. Andrea February 3, 2008 at 8:11 pm #

    Hi Eric,
    I am also trying to rectify the green water situation in my tank and would like your suggestions.
    I have a 20-gallon tank with 4 pencil fish tetras and 4 black skirt tetras. I also have plastic plants and decorations.
    I have tried many of the ideas from the above comments but nothing has improved, even in the slightest.
    Before the water turned green and was just cloudy, I treated the tank with B-Clear three or four times. When nothing improved, I completely restarted my tank by changing 100 % of the water, washing the gravel, and scrubbing everything inside the tank. I also changed my entire filter.
    After two days, the water was completely green again. I cut down to feeding my fish once a day instead of twice. Then, I covered my tank with a dark, thick cover for three days and left the light off as well. A few days later, I turned off my heater for a day. Nothing has worked!
    Now, I just don’t know what else to do. Changing my water by 20% each day will not be possible as I work full-time and have two small children – need I say my time is limited!
    Any suggestions??? I would really love to SEE my fish again!
    Andrea :o)

  46. Eric February 5, 2008 at 9:13 pm #

    Hello Andrea,
    What are your ammonia, nitrite, and nitrate levels like? The DEEP cleaning worries me. The comment above yours here address the 100% change. I would recommend the same thing. I’d guess your levels are not doing so well. You’ll want to get some bacteria starter and start the cycling over from scratch if you want to keep your tank going. The unfortunate part is that this does take time. Frequent, partial water changes will be necessary to keep things in line for probably a month or so. There really isn’t a quick fix to cycling. Once stable, you can relax things.

  47. Andrea February 8, 2008 at 9:58 pm #

    Hi Eric,

    The ammonia, nitrite, and nitrate levels are usually normal or maybe a little high. I haven’t checked them in a few weeks though.

    I’ve always used a bacteria starter every time I did a water change. But, what do you mean by “start the cycling over from scratch”?

    How frequently do I need to do the partial water changes in my case? And how do these water changes fix green water? Everytime I did a water change in the past, the tank stayed green.

  48. Eric February 12, 2008 at 8:03 pm #

    By “start the cycling over from scratch” I was just referring the good bacteria needing to be built back up again. By changing all of your water at once and completely cleaning your gravel, you lose a lot of it. You’ll still have some in your filter if you didn’t also replace it.

    The water change schedule I suggest is 20% a day until it completely clears up. This shouldn’t take longer than a week. After that you can back off. They just need to be aggressive to really fight the problem.

  49. lisa February 17, 2008 at 12:15 am #

    I was wondering if anyone new how to clean a fish tank that’s cloudy green with out empting the entire waster tank?

    Please send me a mesage as soon asap

  50. Eric February 17, 2008 at 4:41 pm #

    Hi Lisa,
    Take a look here for water changing tips.

    The comment right above yours also talks about aggressive changes to get the problem quickly under control.

  51. steve February 19, 2008 at 6:02 pm #

    hey eric the water tempature was up to about 85 If not hotter

  52. Eric February 21, 2008 at 7:25 am #

    Hey Steve,
    Thanks for your story! Yeah, that makes sense. That is pretty hot for a tank to be kept for any length of time. The high temp could really start speeding up the growth.

  53. Sandy K February 23, 2008 at 6:35 pm #

    Eric I have also a GREEN tank. I have changed the water, cut the feeding, cut the light and still I have this GREEN tank. I can not enjoy my fish. I have also used the Algae chemicals. This has been going on 6-8 months. I also put a total of 3 filters in my 20 gal tank. Ths filters I change and they are full of the green algae. Any further tips?

  54. steve February 24, 2008 at 7:28 am #

    Hey sandy do you have a heater in your tank. If so try to unplug it for a day or so and see if that helps at all

  55. Kristy March 7, 2008 at 2:58 pm #

    My brothers fish tank is growing white algae on the bottom of the tank, mostly on the rocks. Can you tell me what might be causing this? He has had the tank for about three years and this is the first time he has had white algae. Thanks a lot!

  56. Eric March 10, 2008 at 8:20 pm #

    It’s most likely a fungus. Have the tank maintenance habits changed recently? Poor water quality is the most common reason for any fungus to pop up.

  57. mike March 18, 2008 at 10:26 am #

    Hi must we off our filter when we are using chemical to treat green water

  58. Eric March 20, 2008 at 8:31 pm #

    Hi Mike,

    Depends on the treatment type. There are a number of treatments available. The instructions should tell you if it’s necessary.

  59. mike March 22, 2008 at 4:26 am #

    Thanks Eric

  60. Lisa March 23, 2008 at 2:15 pm #

    hi i dont have an algae bloom,my water is discolored from a giant piece of driftwood i bought i soaked it changing the water everyday for two weeks i was told that the brown was good for my fish and to let my filter take care of it
    my filter cant keep up … any advice?

  61. Eric March 25, 2008 at 8:20 pm #

    Hi Lisa,

    You should pull it out and soak it separately again. Change the water everyday until the water is clear before putting back in your tank. It won’t just go away on its own. I have had to soak a log for months before!! They don’t tell you that when you buy them do they 🙂

  62. Cathy March 30, 2008 at 4:36 pm #

    I have a ten gallon tank with 5 neon titras and 1 male beta. The water is turning greenish after I cleaned the tank a few days ago. I can’t see any algae in the tank so i am baffled. Should I cover the tank with a blanket for a few days to see if this helps?

  63. Eric March 30, 2008 at 8:31 pm #

    Hi Cathy,
    Getting rid of the light can help if it is an algae bloom. If you use a blanket, make sure you completely block out the light. You are trying to kill off the algae completely. If there is any light, some of it will live. After a few days of noooo light can kill it off.

  64. Cathy March 31, 2008 at 5:51 am #

    Thanks so much Eric! Will try this experience tonight after work and let you know if this experience works! I am so happy to have some advice concerning my fish tank!
    Nova Scotia, Canada!

  65. Jenn March 31, 2008 at 12:26 pm #

    Hiya everyone!

    I had issues with a tank that finally settled down otherwise, and then the algae started to really bloom. I did all the normal steps and was still dealing with cloudy green water, so I noted Gazali’s response, looked up some UV sterilizers, and bought one to put in my tank.

    The dang thing works great!!!!!

    I noticed changes after a few days (even with the larger sterilizer, my tank is 90 gallons so I figure that would make it take longer.) I am now in the much happier position of having to be really careful to thoroughly clean my canister filter once a week to get the dead algae out. The fish are much, much happier too.

    I’ve been running the UV sterilizer for about a month, maybe 6 weeks. It’s done wonders for the clarity of my tank!

    Just as a note: I was worried that I’d have to up the feeding tablets for my plecostemus. But in the tank the size I have and three huge goldfish, there is still plenty of algae for my pleco to eat it seems. So I’ve been able to give him his normal amount of food and he’s even much happier as he gets the algae he likes off the aquarium walls.

    I hope this helps for some folks – I found that it is working great and I’m thrilled I don’t have to put additional chemicals into my tank.


  66. Andrea April 4, 2008 at 10:20 pm #

    Hi Eric,

    I have had very green water in my 20-gallon tank for about 5 months now. I only have 7 tetras. I have tried just about everything but nothing has worked.

    Recently, I did 25% water changes every day for 10 days. The water got clearer but became green again as soon as I stopped the daily water changes.

    Then, I took out all the plastic plants and decorations, changed the filter, added some extra-fine floss to the filter, added phosphate remover, did not feed the fish for 5 days, had no artificial light for 5 days, and covered my tank for 2 days (with a very thick blanket). I still have green water.

    When I last got the water tested, the phosphate was a bit high but the nitrite, nitrate, and ammonia were all fine. This is why I added the phosphate remover. I still have green water.

    I was told by a fish store not to put anymore bacteria supplement in my tank (since it is not new) and to only rinse the filter when it is green but not when it is brown. I was also told to rinse the extra-fine floss as soon as it was dirty. What is your opinion?

    I now leave the artificial light on for only 8 – 10 hours a day. I did a 50-60 % water change tonight and the water is clearer. But, I am sure it will get green again in a day or two.

    Any more suggestions? I am seriously thinking about throwing out my tank at this point! I am really regretting ever buying it…

  67. Eric April 5, 2008 at 9:29 am #

    Thanks for the story Jenn!

  68. Eric April 8, 2008 at 6:22 am #

    Hi there Andrea,
    Sound like you are pretty frustrated. I would be too after all you have been through. I would suggest a UV sterilizer at this point. It WILL clear it up. Many do not want to spend the money, which is why they are not suggested that often but they work. If you are considering throwing in the towel I’d say you are to the point where you should pick on up.

    One other suggestion. Is your water them up too high? Many times this really fuels algae growth as well and it’s the only thing I didn’t see in your notes.

  69. Andrea April 8, 2008 at 12:42 pm #

    Hi Eric,

    The water temperature is always 78 degrees.

    What was your opinion about the bacteria supplement and rinsing the filter?

    Do you think Algaefix or a live plant will help or should I go straight to the UV sterilizer?

  70. Eric April 8, 2008 at 9:04 pm #

    Hey Andrea,
    You really don’t want to rinse the filter with what you are going through. You just wash away good bacteria that is your only hope for working on the algae. Rinsing the floss is a good idea. The bacteria supplement should be helping the cause so I would continue that. The plant won’t really help. I would go straight to the UV sterilizer given you frustration with it. Chemicals seem to have mixed results and many times do more harm than good.

  71. Brenda April 9, 2008 at 11:05 am #

    i have one 20 gallon tank with one koi fish and one comet, one ten gallon tank with two comets and one shubunkin, and a ten gallon with assorted tropical fish. even with frequent water changes, the water is constantly green, yellowish in the tropical tank. im so concerned about their health. what can i do to make sure that my fish are okay, and keep the water clean?

  72. Joe April 12, 2008 at 6:10 am #

    I have a 30 gal reef with 2x 96watt p.c light. 1 light is 10k and the other is blue actinic. I have a green algae bloom and I cant see through the tank.I have done several water changes apx 20% at a time. I have 2 fish 45lbs live rock and live sand. Will chemicals kill the coraline algae on the rock and damgae any of the living stuff on the rock?

  73. Eric April 15, 2008 at 6:15 am #

    Hi Brenda,
    Overfeeding, too much light and/or a heater cranked up too high are the biggies. The most common I would say being overfeeding. Do some major water changes, 50% a couple of times a couple days apart to clear it out. While doing this, make sure you are paying serious attention to feeding. If any food ends up on the bottom of the tank, you’ll want to keep cutting back until it doesn’t.

  74. Jean April 16, 2008 at 8:51 am #

    My tank’s ph was too high and I just read that adjusting ph (down) can add phosphates-which in turn increases algae growth. Which explains why my tank is now green. You just can’t win…

  75. Eric April 16, 2008 at 9:22 pm #

    Hi Joe,
    Before using chemicals you should probably kick up the water changes. Those chemicals can do some damage. Try a 50% change every couple days until it’s gone. Make sure you are adding bacteria stater with changes. This will clear it. You then need to find the cause. Look for the signs above to keep it from kicking back in to gear. Overfeeding is the most common. Try cutting back to a minimum for awhile.

  76. bobby April 19, 2008 at 1:26 pm #

    My 30 gal. tank’s water is continually green, regardless of weekly 30% to 50% water changes. I’ve had the tank for over a year now, and have tried the use of algeafix, but it only works for a day or two before the green terror returns. Is it safe for my fish and live plants to add a higher does of this stuff, or to try the no sunlight approach?

  77. Eric April 23, 2008 at 9:21 pm #

    Hi Bobby,
    I wouldn’t add any more Algeafix than suggested. Try 50% changes a couple of days in a row. That will get most of it out of your tank. Then you should work on the cause. Overfeeding is the most common. Make sure you are feeding no more than they actually eat without waste. A UV sterilizer is another alternative. It will kill it off, but they aren’t cheap. Take a look through the comments about UV sterilizers above.

  78. CRISSY May 14, 2008 at 7:39 am #


  79. Matt May 14, 2008 at 8:43 am #

    We had cloudy/milky water and when we cleaned the tank the water came out looking like someone had dropped green food dye in it. We tried lots of things…i.e. algae fix (which killed our fish), covering the tank from outside light, water % changes everyday and such. We read on your site about UV sterilizers and this was the only thing that worked. Now our tank is gorgeous! And the best thing is that our fish seem to be enjoying it much more. Thanks for the help!

  80. Eric May 16, 2008 at 5:43 am #

    Given the full water change you did, you should now be looking for the source. If it keeps coming back, there is something that keeps re-causing it. Changing feeding habits(too much), moving the tank (now in sun), or bad heater raising temp too high can even fuel algae growth. If you can’t narrow down the cause, a UV sterilizer will kick it. Check out Matt’s comment above.

    Great to hear Matt!

  81. Eric F May 19, 2008 at 10:33 pm #

    Hi Eric, I am stumped with a stump sized piece of driftwood, my water is a tanish, yellowish, rustish color? I recently moved from a 35 to a 110 cube I used 10% of my 3 year established water from my 35, some bacteria starter, and some starter fish for two weeks before making the transfer.My equipment is a 700gph in the sump a u.v.filter 50/50 lights and a small alkilie light. I put the driftwood in to DROP the ph of my 9.8 rich wellwater for Cichlids and the next day , bam. I change out the carbon once a week to offset the driftwood/peamoss effect, but to no avail .If you stuck a beer can in a pantyhose thats what my filtermedia bag of carbon looks like. The fish are happy the ph is good… but my wife keeps saying how ugly yellow it is .And I cant hide it since I built into the stone wall of our new living room for everyone to see. help

  82. Ryan May 25, 2008 at 3:34 am #

    Hey, just wanted to say thanks after reading lots of posts and clearing up my water after several weeks of frustration. I did a series of 30% water changes every other day for about a week while I had my tank completely covered and it worked great. After three for four days saw big changes. My cichlids are as happy as I am.

  83. Eric May 27, 2008 at 6:07 am #

    Hello Eric F,

    Take a look here.

    Try taking the driftwood out, soaking and changing the bucket water until your the bucket water is clear. That will clear up the problem root. Next you just need to do some big water changes, 50% or so every few days until the reddish water dilutes and goes clear. With your filter, gravel and such established, your bio-filter should be fine.

  84. Eric May 27, 2008 at 6:08 am #

    Hey Ryan,
    Thanks for reporting back! Great to hear!

  85. Liz May 31, 2008 at 11:40 pm #

    Hi all,
    I am not one to advocate aquarium chemicals normally, but I have to put in a plug for algaefix here. I had a green water algae bloom and tried everything and nothing did a darn bit of good. I added some algaefix as a last resort and my tank was crystal clear in two hours. Two hours, people! After weeks of green pea soup water.

    Two pieces of advice if you use it: First, stuff a bunch of floss in your filter before you add it so that the floss will catch all the dead algae and it doesn’t all just end up on your gravel. Second, don’t do a big water change right away. I was worried about having those chemicals in my water after the algae cleared up so I did a 50% water change right after it seemed to have done its job. The green water came back in a few days because I hadn’t really killed it all. Then I added algaefix again and didn’t change the water right away and it worked for good. My fish and plants were fine and my nitrites and ammonia were still zero so it didn’t harm my biofilter.

    Also, I have heard that if you block all light from your tank you will cause the algae to stop making chlorophyll, which means it won’t look green anymore. But it will still be alive. Then when you think the problem is solved and you let the light in again the algae restores its chloropyll production and you’re right back where you started. So beware of that. Obviously you can’t block light if you have plants in your tank, too. That’s another pitfall of that method.

  86. Tripp June 1, 2008 at 10:41 pm #

    Hi All
    I have a 30 Gal plant tank. About 3 weeks ago i got green water so i took the measures to clear it up and it worked. But now my water is cloudy. My fish are fine and my plants look like there on steroinds. Any ideas to what i might can do to clear the water up.

  87. Eric June 3, 2008 at 8:40 pm #

    Hey Tripp,
    I would start looking for the cause. Overfeeding? Could you have a lot of plant decay that needs better gravel siphoning?

  88. Stacey June 22, 2008 at 12:39 am #

    Hi, I have a 24 gallon AquaPod that I’ve had since January. Everything was so perfect and all of a sudden things got green. I tried using algaefix and using Algone and so far nothing is working. I have read your comments about covering the tank and giving it absolutely no light, but my fear is for my corals. I have a reef tank and I’m just wondering if it is safe to completely cover the tank, and if so, how long should I try that for? I’m so scared that everything is going to die soon since I cannot see into my tank. Thank you for any help!

  89. Eric June 25, 2008 at 6:04 am #

    I would start with some serious water changes. 40%-50% every few days if you are worried about it being that severe. That’s going to be your quickest way if your worried about your corals. A few days without light shouldn’t harm them if your want to try the no light method. Algae is a much smaller organism and is affected much faster without light. I still suggest the big water changes if you want to quickly clear your tank. Then you can work on the root causes.

  90. Jennifer July 21, 2008 at 6:23 pm #

    Hello, Eric.

    A few days ago, we set up our fish tank. It is a 20-gallon tank and we have 3 pool comets.

    We filled it up late in the afternoon, treated it with a neutralizer and by MORNING, it was green and so cloudy we can barely see the fish. The pad in the brand-new filter was covered in green algae. In a panic, I confess we reached for a chemical treatment, and it’s not touching it.

    Light and feeding are NOT an issue here. We feed only what they eat and the shades are kept drawn.

    Any ideas on what is causing this and how to fix it? I’m worried about the fish!

    Appreciate your assistance.

  91. Eric July 24, 2008 at 6:32 am #

    Hey Jennifer,
    This sort of “bloom” happen with new tanks without any established bacteria. It’s really not toxic for your fish but just something you don’t want to look at. With a 3 day old tank, you might go ahead and start form scratch since there is nothing really established yet and with the chemical treatment. Watch for Nitrates as you go. Also, get yourself some bacteria starter to help get things going. Take a look here for a look at the cycling process.

  92. Tyler July 24, 2008 at 11:30 pm #

    My tank turned green all the way back in December. I didn’t know why or how but it did. I used algae remover, that was a temporary fix as it just came right back. I tried putting a blanket over my tank for a whole week without feeding my fish or even peeking in the tank. That made it translucent, but green never-the-less.
    I took my tank down, new water gravel sponge, everything! I even reduced the lighting in the tank by half. Some how some way in found it’s way back to green. During water changes I noticed that the water coming out was more concentrated than the water in my tank. So I scrubbed the bottom of my tank and the water started to clear up again. However once again it grew back. I tried raising the temperature and lowering it, neither one helped.
    However miraculously one day out of the blue, well my tank turned BLUE!!!! I really don’t know why or how or if it will ever go back to green. But I’ve been green free for almost a month now, even when I leave the light on for 8 hours a day!!
    I now have live plants, elodea and sword, and my fish are healthier than ever!!

  93. Eric July 28, 2008 at 5:54 am #

    The great mystery huh Tyler. As long as it’s back to clear, I wouldn’t ask why 🙂 It’s probably just that any excess nutrients finally thinned out to nothing. With your recent attention to it, I’m sure the root cause of it was improved. Algae starve, clear water 🙂

  94. Dutch July 31, 2011 at 4:39 pm #

    UV is definitely the way to go. I had a persistent algae problem until I went with UV and it cleared up completely in less than a week.