Aquarium Water Changes, Are They Really Necessary?

They sure are.  Well, if you want to maintain a healthy tank they are.  Frequent, partial aquarium water changes help ward off disease and keep your chemical balance in check.  Aquarium water changes are the perfect supplement to your filter to maintain the ideal water environment for your fish.  The three questions that come up most often about water changes are how much, how often and just plain how?

How Much Water Should I Replace?

25% of your tank water should be changed at each interval after you get beyond the initial cycling period.  This is the standard for most fish tanks.  Larger fish tanks can tolerate a smaller percentage change of 10%-15%.  This is because water changes happen slower in a larger tank.

How Often Should I Change My Fish Tank Water?

Aquarium water changes are recommended to be done once week no matter the tank size.  If you are having chemistry problems, you may need to do water changes more often than this.  Water changes are the best way to quickly reduce high chemical levels in your tank.  Realizing that once a week might be a burden on some, it would be to your advantage do them once every two weeks at a bare minimum.  Without this your tank can quickly get away from you.

How to Do Aquarium Water Changes

You’ll need a bucket, siphon hose, and siphon footer.  One end of the hose goes in the bucket.  The other end goes in the tank with the footer on it.  You just drag the siphon over your gravel and plant leaves once you get it started.  Vacuuming the plants is important because of the algae that can quickly build up.  The gavel of course, racks up a bunch of debris.  Gravel will start to make its up the tube, but don’t worry.  The weight of it will bring it back down.  This just leaves the debris, algae and water to get siphoned out.

Some Other Notes

Pretty simple isn’t it.  Don’t forget to add conditioner to the water you add back to your tank.  Also, you’ll want to get your water temperatures close to the same before adding it to your tank so you don’t shock your fish.

How often do you do water changes?

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32 Responses to Aquarium Water Changes, Are They Really Necessary?

  1. adge December 7, 2007 at 3:02 pm #

    can i say, i found your site by accident whilst browsing on the internet. I found it most interesting and gives very good informtion, so now i use it instead of
    reading it up in my books.

    very good site

  2. Eric December 7, 2007 at 4:07 pm #

    Thanks so much Adge! Glad you enjoy!

  3. Zore January 10, 2008 at 10:29 pm #

    I agree Eric. This is one of the best and most educated websites about aquarium life I’ve visited so far. I learned alot. Thanks.
    have a do you have this much information about fish?

  4. Eric January 12, 2008 at 3:44 pm #

    Hi Zore,
    Thanks for the kind words. Personal experience and A LOT of research. I really just love the hobby!

  5. Manny January 27, 2008 at 7:25 pm #

    Same here Eric. Great informative site. I started with some goldfish for my kid and after a bad experience have now turned into a hobbyist. I would like to thank you. I just ran into your site doing a search on Google and it is super informative and easy to read. I am cycling a new 20 gallon tank and made a comment on a previous post. I am hoping with the information on this site I will be more succesful on this try.

  6. Eric January 28, 2008 at 10:49 pm #

    Thanks Manny!

  7. Peter March 2, 2008 at 12:27 am #

    Hi Eric,

    I really appreciate your guides in maintaining a disease free aquarium. I myself has found great joy in my aquarium and ever since i have one, I forgot to turn on my television. It can really become addictive and very informative at the same time.

  8. Eric March 6, 2008 at 6:54 am #

    The hobby can be a lot of fun and certainly addictive. Glad you enjoy!

  9. Ruairi March 10, 2008 at 12:54 am #

    Hi, I just set up a 20 gallon tank and got some platies (5) to start the cycle i have a whisper power filter (Up to 60 gallon tank) and about 8 real plants. 1 of the platies died already after 2 days, how often should i clean the water or do you have any tips about keeping them alive Thanks any help would be good help as i’m a new aquarium owner

  10. Eric March 10, 2008 at 8:34 pm #


    I suggest 20% water changes every 2-3 days given your situation. 5 fish is just too many to start a tank. The ammonia/nitrite/nitrate levels spike too quickly and kill fish. You’ll want to get yourself a test kit /strips and test daily for the nest couple weeks. If the levels are high, do a partial water change. Take a look here for more cycling tips.

  11. Ruairi March 10, 2008 at 8:41 pm #

    Thanks Eric i changed the water today about 20% and will keep doin so but should i dig in to the gravel with the gravel vac or just stay above it, i read that you should try not disrupt the the gravel is that true and thanks for your help

  12. Eric March 11, 2008 at 8:27 pm #

    Yeah, you will want dig into the gravel. Doing that gets rid of the waste that causes the spikes. Your gravel does build up the good bacteria that you want, but partial water changes won’t disrupt things. To your point, it is a bad idea to take it out and clean it completely.

  13. Ruairi March 11, 2008 at 11:12 pm #

    Thanks again Eric, you and this site have really helpful THANKS!!!!

  14. Rick March 25, 2008 at 12:19 pm #

    I recently performed a water change 5-6 weeks after setting up my tank. My dalmation molly gave birth and we moved the 20 or so babies and mommy, seperated though, into a 29 gallon tank. I realize when they reach adult size this tank is probably too small for them. However, that is not the issue. I did a 25% water change for the first time and when I did this, I think I unleashed all the ammonia and nitrites that were in the gravel when I siphoned and cleared the gravel. I have performed at least 5 other water changes in as many days and I can’t get my nitrite levels below 3 ppm, according to my test kit. I tested the water from my faucet, and it doesn’t register any nitrites. Should I perform a 50-60% water change to try to dilute the levels even more? The mother fish was the first to show signs of the levels, by being lethargic and :gasping:. However, she is not a lot more active, and not gasping all that much, but the levels are still uncomfortablely high I’m sure. But the babies don’t show any signs of being affected.

    What would be the best way to take care of this issue?

    Thanks for all your help and very informative website.

  15. Eric March 25, 2008 at 8:38 pm #

    Hey Rick,
    It sounds like you could have siphoned out a lot of the good bacteria from the gravel if you really dug into it. A really aggressive change can sometimes do this if bacteria is not established somewhere else in the tank, like the filter. This is probably the reason for the spikes. It’s a tough situation. Water changes are a good thing, but early on, you have to be careful.
    You will need to keep doing changes to keep up with the spikes. I would try 25% every day or two with a close eye on levels. Just drag the siphon over the gravel in case you dug in good last time. The only risk in the 50-60% changes is that you don’t really give your tank a chance to start rebuilding the bio-filter.

  16. Darci March 29, 2008 at 3:52 pm #

    I added mollies and swordfish to my tank not realizing that the mollies could not take the cycling spikes. Now I have lost one grown up and 6 fry. The 2 mollies left are acting wierd by hovering and one actually at times just floats nose up. My Nitrite levels are high I have been doing 25% water changes its not helping. I have thought of adding a chemical called Prime to lower the levels. Should I do this? And how often should I keep changing the water?

  17. Eric March 30, 2008 at 8:25 pm #

    Hey Darci,

    Prime can help get your fish through this. Chemicals should be a last resort. Since your fish seem to be stressing severely, I would suggest it. It will help turn your harmful chemicals non-toxic. As for the water changes, as often as needed. Test and change if necessary. I t could be up to 25% once a day if you have a number of fish in there too early.

  18. Pam April 2, 2008 at 12:05 pm #

    HI I have a 65 gal tank and cant grt rid of the bactrial bloom i have Will cant say that It was cloudy for a while just changed the water and rinsed the sponge ,coal, white things out and boy was iot green Ive had the tank sence christmas I dont want to use any chemicals PLEASE HELP ME get this under control there is alot of light in the house is this keeping it from getting better?

  19. Darci April 5, 2008 at 11:38 am #

    Thanks Eric. Though my mollies did not survive I have been able to get through the cycle. We made it through with the swordtails , 2 molly babies, 2 snails and one Pleco. I have added 2 tetras and things are going well. Thanks for your help!

  20. Natascha April 6, 2008 at 8:55 pm #

    The past several times I’ve done a vacuum and water change, I get what I assume is a bacterial bloom. I’ve had the tank up and running for sevral months. Is it possible for the bacteria to use up all the oxygen? I lost a frog last time, and my rubber lip today. My other fish looked like they were trying to breathe air at the surface. How do I prevent this from happening again?

  21. Eric April 8, 2008 at 8:34 pm #

    Great to hear Darci! Thanks for checking back!

  22. Eric April 8, 2008 at 8:53 pm #

    This can happen when your good bacteria is disturbed too much and is common with very aggressive water changes. Try just sort of dragging your siphon over the gravel going forward and see if they go away.

  23. Michael Myles April 13, 2008 at 8:01 pm #

    Hi eric, im at present treating my goldgish,comets,shubunkins for ick, i purchased a heater and set it at 82f, how long should i do this for? and when i do a water change do i add salt everytime

  24. Eric April 16, 2008 at 9:28 pm #

    Hello Michael,
    You should keep the temp up until the signs of Ich start to go away. You would want to add salt every time you do water change. Otherwise the concentration weakens and the organism can survive.

  25. Jean April 29, 2008 at 10:15 am #

    I have 2 tanks and my ammonia levels are off the charts on both. I did a 30-40% change in both and added ammo-lock as well as the fizzing tablets to try to get rid of the ammonia and nothing has worked-still off the charts. The fish seem to be alright, although one of my oscars won’t eat normally. I am worried that maybe the filter has stopped working? I have been adding the bacteria and all that regularly. Any suggestions would be greatly appreciated.

  26. Eric May 4, 2008 at 3:08 pm #

    Hi Jean,
    If your ammonia is VERY high, you should be doing big water changes. I would suggest 50% every day or two until you see it under control. Make sure you are not just taking water but siphoning the gravel and getting all of that waste out of your tank. That’s the real source of your problem. Also, make sure you change your filters. Many times they collect too much waste if they are not changed regularly.

  27. Jean May 7, 2008 at 6:34 am #

    Thanks, Eric.

    I have been doing some large water changes and siphoned the gravel and it seems to have lowered the ammonia to 4ppm, a huge improvement. The girl at the pet store told me not to rinse the filter of waste, just replace it every 4 weeks. Is this good advice?

  28. Eric May 7, 2008 at 9:51 pm #

    Great to hear! I think it’s just easier to replace them. Usually you only get one good cleaning, then they start falling apart. While they can collect some good bacteria, they mostly collect harmful waste.

  29. Aaron June 17, 2008 at 8:53 am #

    Very informative site, I just bought a new aquarium and at first the water was very cloudy. I was going to change all the water when I found this website and saw the information about the good bacteria. I left the aquarium for a couple days more and now the water is crystal clear. Thanks again and keep it up.

  30. Nancy July 23, 2008 at 9:24 am #

    This website is one of the best online regarding simple aquariums. Should I ever be cleaning out my whole aquarium and washing the stones? They seem to hold so much “crud” – is it good to wash them out?

  31. Eric July 25, 2008 at 6:25 am #

    Thanks Nancy! You don’t want to clean the whole tank out. You have to start cycling from scratch. A lot of that crud helps keep you tank in balance. After you have an established tank, it’s OK to clean off stones and other decorations so they don’t look so bad. You just don’t want to get too aggressive with the cleaning. Your water, filter, gravel, and decorations all hold good bacteria.