Regulating pH in an aquaponics system

How does pH work ?

For fish, pH influences two essential things : physiological stress and ammonia toxicity. The higher the pH, the more dangerous the toxic form of ammonia (NH₃) becomes. This means that at a high pH, ​​the same level of ammonia is much riskier than at a lower pH. This is a point that is often underestimated.

Nitrifying bacteria, which convert ammonia into nitrites and then into nitrates, have an optimal operating range. When the pH drops too low, their activity slows considerably. The system can then accumulate ammonia or nitrites, even if the biofilter is properly sized.

Plants, for their part, depend on pH to absorb nutrients. At high pH levels, iron, manganese, and sometimes phosphorus become less available. At pH levels that are too low, other mineral balances are disrupted, which can affect growth or fruit quality.

In most aquaponic systems, the most functional range is between 6.4 and 7.0. This zone allows for efficient nitrification while maintaining good nutrient availability for plants.

A pH around 6.6 to 6.8 is often an excellent compromise. It limits iron deficiencies, protects fish from excessive ammonia toxicity, and allows the biofilter to function properly.

It is important to understand that stability is more important than precision. A system stable at 6.4 will often give better results than a system fluctuating between 6.8 and 7.4. Living organisms tolerate slightly imperfect pH well, but very poorly frequent variations.

In a mature aquaponic system, a gradual decrease in pH is normal. It is directly linked to nitrification. With each transformation of ammonia into nitrates, hydrogen ions are released, which acidifies the water.

The more productive the system (more fish, more food, more active bacteria), the more pronounced this acidification will be. If the initial water is low in minerals, the pH can drop quite rapidly because the water lacks buffering capacity.

This is why many systems seem stable at first, then encounter problems after a few months. The nitrogen cycle is then fully active, but the pH is no longer sufficiently maintained.

Measuring pH is only meaningful if the results are interpreted over time. Drop tests are reliable for everyday use. Electronic pH meters are very accurate but require regular calibration.

The key is not the isolated value, but the trend. A pH that slowly decreases each week indicates that the system is using up its natural buffer. A pH that drops sharply often signals a more serious imbalance.

It is recommended to measure the pH at the same time of day, ideally once a week in a stable system, and more frequently during start-up or after a major change.

In most cases, a pH that is too low is the result of active nitrification combined with poorly buffered water. In other words, the system is functioning… but it lacks the mineral reserves to absorb the acidity produced.

In this situation, the most effective solutions rely on the gradual addition of carbonates, particularly calcium and potassium. These elements slowly raise the pH while strengthening the mineral structure of the system. The goal is not to correct the problem once, but to maintain the pH over time.

Hard, calcareous water naturally maintains a high pH, ​​even if the system is functioning correctly. The most consistent solution is to improve the quality of the makeup water, for example by mixing it with softer water.

Managing pH without knowing other water parameters (such as hardness or nitrates) is like driving blindfolded. Paper test strips are often inaccurate and difficult to read. To make the right decisions, you need to visualize what's really happening in your water using reliable colorimetric tests (drop tests).

The essential investment is a Complete Analysis Kit (like the JBL one). Unlike cheap electronic gadgets that malfunction, these laboratory tests give you reliable readings for pH, as well as for Iron, Nitrates, and Hardness.

This is the only way to know if your pH drop is due to active nitrification (good sign) or a water hardness problem (bad sign).

When the pH drops, the common mistake is to use generic chemicals. In aquaponics, we have access to a much more effective tool : Potassium Bicarbonate (KHCO₃). This product, available as a crystalline powder, is the preferred solution for professionals because it does more than just raise the pH.

Its primary role is to raise the carbonate hardness (KH), creating a buffering effect that stabilizes your water and prevents sudden fluctuations that can be harmful to fish. But its main advantage lies in its formula : it provides your plants with potassium (K). This is the most essential nutrient for the flowering of tomatoes and fruiting vegetables.