pH and Water Hardness in your Aquaponics System

Most of us aquapons know that the health of all the creatures in our systems (fish, plants, worms, and bacteria) depends on proper pH. We also know that we are targeting pH in the 6.8 to 7.0 range but that we don’t have to worry about adjusting it until it goes down to 6.4 or up to 7.8. We also know that the best way to lower pH is with an acid, and that the best way to raise it is with carbonates or hydroxides. We know that rapid changes in pH can be very stressful to fish. And we know that the pH will probably decrease over time because the nitrogen cycle produces an acid (nitric). And we know that pH and water hardness are related.

But often, knowing all of this and applying it to our systems are two different things. At The Aquaponic Source, we handle questions and concerns each and every day about pH and water hardness. Generally, the questions fall into one of two categories:

  1. – The water out of my tap is a pH of 8 (or more) and I can’t seem to lower it reliably. What should I do?
  2. – The pH in my system is dropping constantly and I have to add something to raise it almost daily. Is this normal?

Most pH mysteries in aquaponic systems boil down to how ‘hard’ the water is that we put into our systems. This ‘hardness’ dictates the buffering capacity of water… Let me explain.

Water from most sources has some level of mineral salts dissolved in it (purified water such as distilled or RO [reverse osmosis] filtered are clear exceptions). Among these dissolved salts are certain specific minerals that strongly affect your water pH. The concentration of these minerals in your water is often described by the term ‘hardness’, the higher the concentration of these minerals, the ‘harder’ your water is. There are two types of hardness in water: ‘carbonate hardness’, (KH), which is sometimes also referred to as the water’s ‘buffering capacity’ or ‘alkalinity’ (not to be confused with an ‘alkaline’ solution which would have a pH greater than 7) and ‘general hardness’ (GH) which refers to the concentration of calcium and magnesium ions in the water.

The general hardness of the water affects pH but it is the buffering capacity (KH) of your water that is the more critical pH factor. This buffering capacity acts like an invisible sponge that soaks up whatever acid or base is in your system, or that you add to your system, until the buffer is ‘used up’. With this sponge-like behavior in mind, imagine trying to adjust your pH. Let’s say you have a pH of 8.0 in your aquaponics system and you would like to bring it down to 7.0. You start adding our AquaDown pH Lowering Agent, and adding, and adding, and little or nothing changes. And then all of a sudden the pH plummets. What has happened? You had a strong buffer (meaning there was a lot of KH) in your system which you eventually ‘overwhelmed’. Another way of thinking about this is that you ‘filled up’ the sponge.

You can actually measure your KH level and doing so may help you manage your pH. The larger the KH number, the more resistant your system will be to attempts to alter pH. Having a higher KH level can be beneficial in a fully cycled system because the nitrification process produces nitric acid which will persistently drive pH down in an unbuffered environment. A rule of thumb is that a KH of less than 4.5 dH (degree hardness) means that you don’t have much buffering capacity and you should be checking your pH a few times a week.

How do you increase your system’s buffering capacity? When you are fully cycled and your pH drops below 6.8, add calcium and potassium carbonate (the ingredients in our AquaUp pH Raising Kit) on a regular basis once you have fully cycled and start seeing your pH decrease below 6.8. You will find that over time, you will build an excellent buffer and your system should become more and more pH stable.




  1. Tim Kelbert August 18, 2013 at 6:33 pm

    My ph keeps stabilizing at 4 and I tried the ph up agent and it is getting closer to 3. I do not have a kh tool measure, so I am adding some pond water(ph of 9) as evaporation occurs, hoping the level goes up. The plants continue to grow and the fish continue to live, and hope the ph goes up at some time. Is it true that I can keep from feeding the fish for a couple of days without jeopardizing their health? We are newbies and learning lots, but also do not wish to lose anything.

  2. Sylvia August 18, 2013 at 7:11 pm

    Hi Tim. I’m really surprised that your system is doing well at that low of a pH…which makes me wonder about the accuracy of the pH numbers. How are you testing it?
    And, yes, you can get away with not feeding the fish for a few days. The will be fine.

  3. Max September 13, 2013 at 11:23 pm


    My system is about 1.5 month old and I have a problem lowering my pH. After reading your post it makes a lot of sense why when I add some pH down not a lot happens. Actually I can see a drop and than my pH comes back up to 8.2.

    Here is my water data and my 110 L tank has 23 gold fish that I feed 1 time every 2 days:

    pH – 8.2
    NH4 – <0,05 ppm
    Fe – 0,02 ppm (4 days ago it was zero but I added some iron)
    NO2 – 0,02 ppm
    NO3 – 40 ppm
    KH – between 5 and 6 (the test water gets yellow at 5 drops)
    K – between 0 and 5 ppm

    My question how can I safely lower my pH and I am guessing increase my K?

    Thank you for your help.


  4. Sylvia September 14, 2013 at 9:18 pm

    Eventually the nitrogen cycle will get robust enough so that it is kicking off enough nitric acid to lower your pH naturally. In the meantime just keep doing what you are doing, but carefully. It is worse to swing your pH widely than to just leave it at the higher levels.

  5. Max September 16, 2013 at 8:21 am

    Thank you.

  6. Clinton school district September 20, 2013 at 7:59 pm

    Our science class tried starting an aquaponics lab and we had 49 out of 50 fish die in 1 week. We found that the ph level was at 9.0 so we tried to balanc it out using acids and it went back to 7.5 ph within 24 hours it was at 9.0. So we balanced it with vinager, same results. Then we used reverse osmosis and it went down to 7.0 but within 24 hours it was back to 9.0.

    How can We safely lower this ph level?

  7. mel October 20, 2013 at 11:09 pm

    hi, my ph has been running at 7.6 during the start up cycling when the nitrates started to show up the ph went to 8.2. Will it go back down on its own or what should I do?
    There are no fish yet.


  8. Mick Baker November 15, 2013 at 8:29 pm

    In my area of Southern Colorado I have hard water and low ph (below 6). I have used your ‘Aqua Up’ product which helps temporarily raise the ph, but with water replenishment due to evaporation, the ph eventually makes a wild swing low again stressing fish and killing my bacteria. Couldn’t I add a water softener cartage to my Aquaponics water source and eliminate the hard water over time and stop these radical swings?

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