At The Aquaponic Source we help customers with questions and concerns about pH each and every day. Generally, the questions fall into one of two categories:

“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?”
“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” your water is that you put into your system. 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 filtered [reverse osmosis] are clear exceptions). Among these dissolved salts are certain specific minerals that strongly affect the pH of your water. 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 of 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 capacity of 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 an acid…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 using an API GH and KH Test Kit. 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, as you will recall, 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 dKH (“dissolved carbonate hardness”) means that you don’t have much buffering capacity and you may see rapid, frequent swings in pH.