pH value for Aquaponics

The correct pH value of the water in an aquaponics system is critical. If the pH is off too far, the fish can die and the "good bacteria" can be replaced with the unwanted bad bacteria. The plants will also suffer from this change in system health. The pH value shows how acidic or how alkaline (base) the water in your system is. This article is one of the more important ones to understand in our DIY Aquaponics section.

Defining pH ranges

A pH of 7 is defined as neutral. It is neither acidic nor alkaline. If a liquid has a pH lower than 7, it is acidic. If it has a pH higher than 7, it is alkaline.

Ideal pH in Aquaponics

In any DIY Aquaponics system, pH should be kept between 6.2 and 6.8. It is important to keep it slightly below 7, although a pH of 7.1 or 7.2 can be acceptable as an absolute upper limit. A pH between 6.2 and 6.8 allows for optimal growth of the plants (best able to absorb the nutrients) and it is healthy for the fish. Expecially for the intake of Iron and Managese, lower pH levels are essential for plant health.

In a healthy Aquaponics system, the pH should be continually (and gradually) dropping. Depending on factors including the temperature, the pH could drop 0.1 per week (so from 6.6 to 6.5 in one week). It needs to stay higher than 6.2, so adding some margin, when it drops to 6.4 the pH needs to be raised a bit. There is an excellent way to do this. There are three elements that generally do not enter an aquaponics system easilly:
  • Iron
  • Potassium - very important to fruiting plants including tomatos.
  • Calcium
These nutrients can be used to buffer up the pH to a level where we want it to be.

Effect of pH on Toxicity

At a pH above 7, the Ammonia that is given off by the fish as waste can take on a toxic state. Ammonia-nitrogen (NH3-N) has a toxic form at high pH (un-ionized ammonia, NH3) and a less toxic form (ionized ammonia, NH4+).

Buffering up using Potassium Hydroxide

This chemical is very base (alkaline), so it must be handled cautiously (see below). Potassium Hydroxide is a white solid. Add extremely small amounts to your DIY Aquaponics system, and add it to a plant bed that is as far away from the fish tanks as possible. You can place it near the water inlet of the plant bed, while the Potassium Hydroxide dissolves, it is generating a lot of heat. If you only have a very small system, you can dissolve the potassium hydroxide in water in a large bottle, and slowly, over the course of 30 minutes to an hour, pour the bottle contents into the plant bed. This ensures no sudden large increases of the chemicals hit the fish tank (and potentially shock the fish).

Note, that Murray Hallam puts only 2 small, level teaspoons (not tablespoons, TEAspoons) of Potassium Hydroxide in his 12.000 liter hydroponics system. Most home systems will be way smaller than this. For a large sized home system (1.000 liters), only 1/6th of a level tea spoon should be added as an absolute maximum. This would be enough to raise the pH of the system by 0.2 in about a 24-48 hour period. Expect to have to add the Potassium Hydroxide approximately once every 3-4 weeks.

Signs of Potassium shortage

The signs of potassium shortage are most pronounced in tomato plants. Look at the stem, where the tomato plant flowers. If the flower heads break off just at the base of the flower head (and small stems are still connected) this is a good indication of a potassium shortage in your system. Note that there could be other reasons for the flower heads to break off, it could be caused by a lack of pollinization. Check surrounding flowering plants to see if they are being pollinized by the bees.

Careful when using alkaline (base) chemicals

Alkaline chemicals can very severely damage your skin, eyes, etc. Unlike acids however, you probably will not feel alkaline as you would an acid burning your skin. Handle alkaline chemicals with extreme caution, they are very dangerous when used incorrectly. Always use gloves and eye protection - don't gamble this one, damage to your skin or eyes could be irreversible. No aquaponics system should be worth that risk.

Bringing pH down

Using Phosphoric acid to reduce pH

Adding very small quantities (gradually over the course of several days to weeks) of phosphoric acid to the fish tank will bring the pH down. A sudden pH change will be a very effective way to kill your fish, so add very very small quantities over the course of two to three weeks to your DIY Aquaponics setup.

Using Hydrochloric acid to reduce pH

When there are no fish in the system yet, hydrochloric acid can be used to bring down the pH. It is not recommended to use this in a system where the fish are already introduced. Again, add the acid very gradually.

Careful when using acidic chemicals

Acids can severely burn your skin. Always use protective gloves and eye protection when working with acids. Simple and small liquid spills can cause severe burns, scarring and can permanently damage your eyes.