Aquaponics Water Testing – 4 Critical Factors

The pH of a solution is a measure of how acidic or basic the solution is on a scale ranging from 1 to 14. A pH of 7 is neutral; anything below 7 is acidic, while anything above 7 is basic. The term pH is defined as the amount of hydrogen ions (H+) in a solution; the more hydrogen ions, the more acidic.

For plants, the pH controls the plants’ access to micro- and macronutrients. At a pH of 6.0–6.5, all of the nutrients are readily available, but outside of this range the nutrients become difficult for plants to access. In fact, a pH of 7.5 can lead to nutrient deficiencies of iron, phosphorus and manganese.

Nitrifying bacteria experience difficulty below a pH of 6, and the bacteria’s capacity to convert ammonia into nitrate reduces in acidic, low pH conditions. This can lead to reduced biofiltration, and as a result the bacteria decrease the conversion of ammonia to nitrate, and ammonia levels can begin to increase, leading to an unbalanced system stressful to the other organisms. Fish have specific tolerance ranges for pH as well, but most fish used in aquaponics have a pH tolerance range of 6.0–8.5. However, the pH affects the toxicity of ammonia to fish, with higher pH leading to higher toxicity.

In conclusion, the ideal aquaponic water is slightly acidic, with an optimum pH range of 6–7. This range will keep the bacteria functioning at a high capacity, while allowing the plants full access to all the essential micro- and macronutrients.

Ammonia is toxic to fish. Tilapia and carp can show symptoms of ammonia poisoning at levels as low as 1.0 mg/litre. Prolonged exposure at or above this level will cause damage to the fishes’ central nervous system and gills, resulting in loss of equilibrium, impaired respiration and convulsions. The damage to the gills, often evidenced by red coloration and inflammation on the gills, will restrict the correct functioning of other physiological processes, leading to a suppressed immune system and eventual death. Other symptoms include red streaks on the body, lethargy and gasping at the surface for air. At higher levels of ammonia, effects are immediate and numerous deaths can occur rapidly. However, lower levels over a long period can still result in fish stress, increased incidence of disease and more fish loss.

Nitrate is a far less toxic than the other forms of nitrogen. It is the most accessible form of nitrogen for plants, and the production of nitrate is the goal of the biofilter. Fish can tolerate levels of up to 300 mg/litre, with some fish tolerating levels as high as 400 mg/litre. High levels (over 250 mg/litre) will have a negative impact on plants, leading to excessive vegetative growth and hazardous accumulation of nitrates in leaves, which is dangerous for human health. It is recommended to keep the nitrate levels at 5–150 mg/litre and to exchange water when levels become higher.