pH and Water Hardness in your Aquaponics System

By: Tawnya Sawyer

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:pH and water hardness

  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.