Managing pH in the Aquasprouts is just as important as hydroponics and soil gardening, but it is a little bit more challenging in aquaponics because there are three living variables in the ecosystem: fish, plants and bacteria.
Plants like acidic pH (in the low sixes to upper fives), the fish and bacteria prefer a slightly alkaline pH (in the high sevens to low eights). pH management in aquaponics is an exercise in compromise between the two ranges. The ideal target is a pH of 6.8 to 7.
pH Over the Life of Your Aquaponic System
Most water comes out of the tap at pH levels above a neutral 7 depending on where you are in Australia. Melbourne is known for it's great quality water. If your system’s water comes from a well, you will probably discover the same thing, but in this case, it is the mineral content of the water that is keeping your pH high.
Good news—a slightly high pH is usually not a problem because when you start up a new aquaponic system, you cycle it. The cycling process is designed to encourage beneficial nitrifying bacteria to take up residence in your system. This process is entirely focused on the bacteria and the fish, which both prefer a higher pH. This means that water a bit above a pH of 7 actually helps to get your system cycled.
Once the nitrogen cycle is fully established, and you have added plants to your system, you will usually see the pH of your system decrease over time. Why? Because the nitrogen cycle actually produces nitric acid, which will naturally cause your pH to go down.
But there will be times, especially when your system is young, when you will need to act to push the pH down. And in the long run, you will regularly need to raise your pH. We will discuss how to do this safely in aquaponics, but first we need to discuss buffers and water hardness.
Carbonate Buffers and Water Hardness
I help customers with questions and concerns about pH 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 the water in your system is. This hardness dictates the buffering capacity of water. Water from most sources has some level of mineral salts dissolved in it (purified water such as distilled or reverse osmosis-filtered water are 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 in your aquaponic system and you would like to bring it down to 7. You start adding an acid, then adding more, and little or nothing changes until 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, and you eventually used up the capacity of the buffer, or filled up the sponge.
How to control pH?
So what do you do if you need to take the pH up or down in your aquaponic system? To lower pH, I recommend certain acids, such as nitric, muriatic, and phosphoric.
My preference is phosphoric because it is the safest of the three acids (it is an ingredient in cola drinks) and it adds some phosphate into your system, which your plants will like. That said, phosphates can exacerbate an existing algae problem, so if that is your situation you may want to use one of the other types.
You should absolutely avoid citric acid because it is anti-bacterial. Also avoid vinegar because it is too weak—you could be pickling your fish before you see the pH results you are looking for.
To raise pH, I recommend using calcium carbonate and potassium carbonate, and either alternating between them or adding equal amounts of each at the same time. I prefer to use these carbonate compounds instead of hydroxide compounds (calcium hydroxide and potassium hydroxide) for several reasons.
First, they add to the strength of the buffer. Second, they are not caustic and won’t burn your skin like the hydroxides will. Third, they are on the Organic Materials Review Institute (OMRI) list of approved additives for organically certified production.
While critical, pH management in aquaponic systems is actually easy. The pH of your system will probably never cause you a serious problem as long as you make sure you maintain a buffer of 4 dKH or more, and that you check and adjust your pH once a week as necessary.