Why Care About Your Aquarium’s Water Hardness?

Your fish tank’s water hardness may be one of the least important elements of the water chemistry puzzle by itself.  Does that mean you can just forget about it?  Not really.  Let’s learn a little about aquarium water hardness and why you must keep it under control.

What Does It Hurt?

Your aquarium’s water hardness plays a very important roll.  The harder your tank’s water, the more calcium carbonate (CaCO3) is in the water.   More calcium carbonate (CaCO3) in your tank’s water gives your water a higher buffering capacity.  Your tank’s buffering capacity makes it more or less resistant to pH changes.  This is VERY important to your fish.  You see, while the hard or soft water itself may not be preferable to some fish, it’s usually livable.  pH on the other hand is a different story.  If your aquarium’s pH is too high or low it can be very stressful for your fish.  The two are really interrelated.  The most important thing to take away is this.  Hard water is more resistant to pH changes and pH changes are bad.

Aquarium Water Hardness Basics

Here are some basic terms that are related to water hardness.  You’ll want to get yourself a test kit.  Your kit should have these but this will get you familiar with the measurements and terms.

  • DH – Degrees of Hardness
  • KH – Carbonate Hardness
  • GH – General hardness
  • Very Soft        0-4 dH – 0-70 ppm
  • Soft                 4-8 dH – 70-140 ppm
  • Medium         8-12 dH – 140-210ppm
  • Hard               12-20 dH – 210-350 ppm
  • Very Hard      20+ dH – 350+ppm

What’s the Ideal Water Hardness Level?

It depends on the fish species.  Read up on your specific species.  Remember to choose fish that prefer similar environment.  Also, don’t forget the relationship between buffering capacity and pH.  The harder your tank’s water, the more stable your tank’s pH will be.  If you tank’s KH gets into the “Very Soft” area you should be paying special attention to you aquarium’s pH.

How Do I Raise My Tank’s Water Hardness?

  • Filter your tanks water through crushed coral.
  • Add baking soda (NaHCO3) to your fish tanks water – 1 Teaspoon per 26 gallons (100 liters) – This will raise your tanks KH around 2 dH.
  • Add calcium carbonate to your tanks water – 1 Teaspoon per 26 gallons (100 liters) – This will raise your tanks KH and GH 2-4 dH.

What If I Want To Lower It?

  • Boiling your tanks water will reduce its hardness although it’s not very practical.
  • Filtering your fish tanks water through peat moss .
  • Water softening pillows can be found at your fish retailer that uses an ion exchange method to soften your tanks water.

Does your tank have hardness issues?

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25 Responses to Why Care About Your Aquarium’s Water Hardness?

  1. bigfish November 13, 2007 at 9:41 pm #

    Can distilled water be used to reduce hardness?

  2. Eric November 13, 2007 at 10:04 pm #

    Hi There Bigfish,
    Distilled water has a kH of 0. On the surface this might seem like a great way to lower your kH. The problem is that with a kH of 0, if an acid is introduced your water it can send your pH rapidly out of whack. Distilled water is just not very stable, therefore it is not recommended.

  3. bigfish November 13, 2007 at 10:11 pm #

    Thanks Eric

    I have a 29 gallon tank and i replaced 4 gallons of it with distilled water…it didnt really make much difference but my hardness reads at about 240 but everything else in the tank is fine. Is that 240 level ok?

  4. Eric November 13, 2007 at 11:34 pm #

    Your Welcome Bigfish,
    It depends on your fish. You probably just want to do a little research on your fish species. African Cichlid’s for example, thrive in hard water while Discus prefer soft water.

  5. bigfish November 14, 2007 at 12:03 am #

    See the fish i have are all tropical. There is a bala shark, kissing gourani, few mollies, a 12 inch peacock eel9(i am the most proud of that one) and a few other small fish.

  6. Eric November 14, 2007 at 9:30 pm #

    Bigfish, All of your fish should be fine. Your Bala Shark prefers a slightly lower level. As long as it doesn’t get any higher, it should be fine. A Bala Shark’s optimal range is in the medium hardness range.

  7. Lee January 9, 2008 at 1:41 am #

    Very good information, thank you.
    I have recently exchanged four large oscars for 6 small silver dollars. I have a 40 gal tank. With in the first two days all the silver doolars have died. The ammonia levels are very low. GH & KH are fine, nitrite & nitrate levels are ok too. The only thing I have found is the ph is very high. I added baking soda after dilutung it to lower the ph. Could high ph have killed off my fish that fast or might there be something else going on?

    Thank you

  8. Eric January 9, 2008 at 10:02 pm #

    Hey Lee, Very high pH levels can certainly be toxic. Killing them off in the first two days does seem very fast, however. If you can’t find anything else out of line it’s probably the likely culprit if it’s extreme.

  9. carpman March 2, 2008 at 9:17 am #

    I am having trouble understanding levels of water hardness in my aquarium. The readings I have taken show a KH of 50mg/l and a GHof 120mg/l but my colour coded Co2 monitor shows the level is to low. As all the books I read and the test kits I have used seem to use different units ie: ppm; mg/l; german degrees and french etc. I am lost can anyone advise me.

  10. Eric March 6, 2008 at 7:07 am #

    Hi Carpman,

    It can be confusing. German degrees hardness (dH) is the most common measure of gH. ppm is another way to measure gH. They are both measurements of the same animal, general hardness. ppm is also equal to mg/l. So, what’s all this mean? You 120 mg/l says your water is soft.

    Very Soft 0-4 dH – 0-70 ppm
    Soft 4-8 dH – 70-140 ppm
    Medium 8-12 dH – 140-210ppm
    Hard 12-20 dH – 210-350 ppm
    Very Hard 20+ dH – 350+ppm

    Hope that helps clear things up for you.

  11. edele March 27, 2008 at 1:06 pm #

    my ph test is 7.9 i dont know if its high or low first time doing this test my fish r dieing slowly what can i do

  12. Eric March 30, 2008 at 8:16 pm #

    Hi Edele,
    Take a look at this article and the comments. Using peat moss is one of the most common natural ways to lower your pH. 7.9 is high for most fish, but I don’t think that would be the cause of your fish dying. I would look for something else in your test results.

  13. Scott April 2, 2008 at 8:52 pm #

    I need some help. The pH in my tank is around 6.0 and my water is extremly hard, with nitrates very high also. I did a 25% water change the week before and since that time I lost a couple fish so I check the water. I can understand my water being hard but why would my pH be low . Isn’t it generally high with hard water. Why would my nitrate level be high. I vacuum half the gravel every two weeks and have for years. By the way this is a 12 gallon Eclipse tank with two coreys and and three dwarf groumies ( well now 1 )


  14. Fishy April 6, 2008 at 2:04 am #

    Hi there, I think you should do a daily water change of around 20% and constantly check your water condition. Daily water change will help improve your water condition.

  15. Eric April 8, 2008 at 6:00 am #

    Hello Scott,
    You are right in that your harder water does provide a buffer and help keep your pH stable, but that doesn’t always control it completely. It sounds like you have something more going on with our Nitrate levels being high. Have you done something to weaken your bio-filter? Treating with medications or using other chemicals can have this effect even if you maintenance habits are good.

  16. Donna April 17, 2008 at 1:16 pm #

    Hi Eric, I have a potential problem, just tested the water in my guppy tank, (6 gallons) and the readings were: nitrate 0, nitrite 0, gh 0, kh 0-80 and ph 6.8 to 7.2. Is that a problem, the gh and kh haven’t ever been that low. I use filtered well water.

  17. Jean April 18, 2008 at 5:47 pm #

    I’m confused…:-( I have 2 kinds of testing strips: one measures GH and KH, PH, NO2 and NO3, the other strip measures TOTAL HARDNESS, TOTAL ALKALINTY/BUFFERING CAPACITY as well as PH, NO2 and NO3. I have used a water softening pillow and it brought my GH down to around 120. It seemed to have no effect on the buffering capacity or KH. I am confused about the relationship there.

  18. Eric April 20, 2008 at 9:32 pm #

    The GH and KH reading are certainly low. Most tropicals prefer a GH of around 60. Are you sure you got a good reading? As long as your pH stays within that range, your fish won’t be stressed too much. You’ll want to keep a very close eye on your pH as it could change very rapidly without the proper buffering. Also, you should try any of the hardness raising tips in the above post.

  19. Eric April 23, 2008 at 6:04 am #

    Hey Jean,
    Some of this is just plain confusing but it is possible to for your general hardness(GH) to lower while your buffering capacity remains unchanged. KH measures carbonate and bicarbonate ions. While GH is mostly a measure of the concentration of magnesium and calcium ions. Like I said, confusing. Basically, if your fish prefer hard or soft water they prefer high or lower GH. If you are concerned about controlling your pH, you need to control your KH or buffering capacity. I know, still confusing.

  20. Connie April 23, 2008 at 5:14 pm #

    I’m sooo confused. Have 90 gallon tank w/ 6 2-3 inch fancy goldfish. Temp about 74-76 degrees, Ammonia 0, Nitrite 0. I have not tested for nitrates because I do –not regular –but frequent partial water changes. Best I can determine w/ my test kit, the GH is 7 and the KH is 3-4. I interpret that as soft to very soft. The city water is a GH of 3 and KH of 2. My problem is the ph. For many months the ph has been about 7.3 to 7.4. Before that it was too low, so I added some coral gravel. I have not knowingly changed anything. The ph is now testing 8.3 or 8.4. I have recently lost a fish to dropsy and I have another that may be ill. I’m afraid to go for peat moss or other ph lowering methods for fear of having even lower hardness readings. I thought soft water = low Ph. I know the Ph is more important than the hardness, but I hesitate to jump in and start changing things. Do you have any suggestions??? Thank you so much!

  21. Eric April 23, 2008 at 9:48 pm #

    Hello Connie,
    Soft water actually has a lower buffering capacity than harder water. This opens the window for your pH to move, and sometimes move big. You should actually be trying to raise your water’s hardness to make your pH more stable. The problem is that some of the same things that lower your pH also make your water softer, like peat. Either way, pH has the biggest effect on your fish so I would work on that first. Once that is under control, you can slowly work on your GH to give your water stability.

  22. Connie April 23, 2008 at 10:28 pm #

    Thank you! Next question— How do I lower this ph? Should I use peat? Or something else? And how fast can I lower it for goldfish. Then once I get the ph down, how do I work on the GH? Thank you! It is so helpful to have someone to ask.

  23. Eric April 28, 2008 at 6:08 am #

    Hey Connie,
    Take a read through here http://www.fishtanktutor.com/be-careful-adjusting-your-aquarium-ph Using peat to lower your pH is discussed throughout the comments. That’s the fastest and easiest way. It probably won’t take much with your GH being so low. You’ll just have to play with the amount.

    Take a look above for GH raising tips.

  24. kevin June 24, 2008 at 4:16 pm #

    I have a 30 gallon tank with the following fish; 1 common plecos, 2 rainbows, 7 blue neon tetras, and 1 mollie. What I would like to know is would it hurt to put some aquarium salt in the tank and if not how much should I put in. Thanks for any advice.

  25. Eric June 25, 2008 at 9:08 pm #

    Plecos usually don’t do well with salt. Very low levels for medicating have been know to be OK, but it’s probably not worth taking the risk of stressing you fish.