Managing pH is just as critical in aquaponics as it is in hydroponics and soil gardening, but it is trickier in aquaponics because there are three living constituencies to consider in an aquaponic ecosystem: plants, fish, and bacteria.

While the plants generally prefer a slightly 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). Therefore, 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 because municipalities are concerned about the long-term corrosive effects of acidic water so they adjust accordingly. 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:

  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 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.

You can 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, 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 you don’t have much buffering capacity and you may see rapid, frequent swings in pH.

Why Create a Buffer in Your Aquaponic System?

There are a few critical reasons. First, while fish can generally adapt to pH levels outside of their ideal range, they cannot handle dramatic, rapid swings in pH.

I get phone calls every week from customers who are struggling with what appear to be random fish deaths. When we ask them to measure their pH several times during the day, we often see dangerous swings of an entire point or more throughout the day.

We then ask them to test the KH levels in their water and they nearly always report that they have a dKH that is below 4.

Second, maintaining a buffer is critical for bacterial health. If you get to the point where your system carbonates are completely depleted, your system pH can crash (decrease rapidly). If it does, your beneficial bacteria will quickly die and biological filtration will stop entirely.

Third, if you don’t have at least a minimum buffer of 4 dKH established in your system, you will need to manage pH on at least a daily basis and adjust as needed. If there is no buffer to offset the nitric acid in your system, it will simply lower your pH unimpeded. So, not only is a lack of carbonates dangerous to your fish and bacteria, but it will also create a maintenance nightmare for you!

How to Create a Buffer in Your Aquaponic System

Here is what I recommend to create a buffer in your aquaponic system:

  • Get an API GH and KH Test Kit and figure out what your carbonate levels are.
  • If you are above 4 dKH, then you should be fine for now but be sure to retest weekly as part of your normal testing regime. Remember that as your system matures, it will create more and more nitric acid, so your KH levels will drop over time.
  • As you approach a dKH of 4 or lower, I recommend adding potassium bi-carbonate to your system at a rate of 2 ½ tsp. per 100 gal. of system water for each dKH level you need to go up.

Raising and Lowering pH in Aquaponic Systems

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. (See pH Up and pH Down for more information)

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.

For more information on pH in general, check out Perfecting the pH of Your Hydroponic Nutrient Solution. For more tips on growing plants using aquaponics, see Maximum Yield's entire collection of aquaponics articles.