Aquaponics is the convergence of aquaculture and hydroponics. The former pertains to growing of aquatic animals such as fish. The latter, on the other hand, refers to growing crops using water, thus, excluding soil from the process.
Much waste is produced by aquaculture in the form of ammonia which substantially comes from fish poo. Ammonia is toxic to fish and must be removed from the water. This is the reason why aquaculture uses a lot of water in its operations. Constant water change is necessary. Otherwise, the fish end up dead due to ammonia build-up.
There is, however, a more sustainable way of removing ammonia from the system without wasting so much water. This is done through what is called nitrification and this process is the key link which allows for the harmonious convergence of aquaculture and hydroponics.
Nitrification is the process through which ammonia is converted to nitrites. The latter is then converted to nitrates. The process is facilitated by nitrifying bacteria called Nitrosomonas (converts ammonia to nitrites) and Nitrospira (converts nitrites to nitrates). For a time, it was previously thought that Nitrobacteria was responsible for the final stage of conversion, though there is still debate as to which bacteria should be credited.
Nitrates are an exceptional source of nitrogen for plants and, to a large extent, are harmless to the fish. If the nitrate level starts to exceed 150 ppm (though this is still definitely better than having fish-killing ammonia and nitrites in the system), the fish will likely suffer from stress and become vulnerable to illnesses. They may also perish in the process.
Excessive amounts of nitrates could mean that the number of plants in the system is too low. To fix the problem, just grow more crops or, better yet, add one or two more grow beds. If the high nitrate level persists, try harvesting some fish or reducing the total amount of feeds fed to the fish on a daily basis.
Nitrification also produces nitric acid. This is the reason why the pH of an aquaponics system that is working properly will tend to decrease with time. For example, dechlorinated tap water used by this writer for his system has an initial pH of around 8.2. After undergoing nitrification, the pH of the water has dropped to around 6.8 to 7. This is the ideal pH zone for fish, plants, and beneficial bacteria to flourish.
If the pH is not dropping, then the water could be “hard.” Another possible reason is that nitrification process is not happening correctly. It may likely even mean that the opposite, called denitrification, is taking place. Nitrates are converting back to nitrites and then back to ammonia!
While a dropping pH level is a good sign that nitrification is happening, if allowed to trend downward, it may lead to big problems. For example, nutrient lockout may happen, fish and plants may die and the system will crash.
To maintain the ideal pH for the system, some practitioners add calcium hydroxide or calcium carbonate along with potassium carbonate or potassium hydroxide. Besides pulling the pH up, these additives also help supplement at least two of the usual nutrient deficiencies in an aquaponics system: calcium and potassium.