Several times per day we feed our fish a high quality diet so that they will grow quickly and retain optimal health.  They digest this feed, reduce the proteins in the diet to amino acids, and then reassemble these amino acids into their own form of protein for growth.  During this process a nitrogen-rich by-product called ammonia is produced, but this ammonia is toxic to the fish.  Ammonia is transported by the blood from the cells to the gills where it is released into the water by osmosis, preventing its accumulation within the body which would result in the poisoning of the fish.
 
Different species of fish tolerate ammonia differently; at 0.5mg/? trout are already showing signs of discomfort, whereas tilapia and catfish tolerate a concentration of even 5mg/? without problems.  Fry are significantly more sensitive than adults of the same species with a hatchery ammonia level of 0.5mg/? being highly undesirable, even for tough species such as catfish.
 
The concentration of ammonia in fish water is measured by means of an ammonia test kit or a spectrophotometer, the latter rarely being used as it is expensive, slow and the highly accurate reading is seldom justifiable.  Test kits are inexpensive, quick and easy to use, and provide an answer that is accurate enough to serve as an early warning system should the water borne ammonia concentration approach an undesirable level.
 
In truth, what we measure is not ammonia but the less toxic form ammonium.  The ratio of ammonia : ammonium in water is a function of temperature and pH, with an increase in either resulting in a swing towards the more toxic ammonia.  As such, it is important not to farm fish at temperatures or pH levels than are higher than is optimal for the species being cultured.
 
In cages the ammonia released by the fish washes out of the cage with water movement, and thus never accumulates to problematic levels.  Earth ponds contain low numbers of fish and are fertilised weekly to stimulate the growth of the feed chain within the ponds, and as such ammonia in a correctly stocked pond is not a problem.  Under recirculating conditions the fish are stocked at high densities and the water continuously recirculated through filters and back to the fish, allowing for rapid accumulation of ammonia to toxic levels.  To avoid ammonia build up in a Recirculating Aquaculture System (RAS) we make use of a biofilter which is colonised by desirable bacteria that convert the ammonia via nitrite (also toxic) to nitrate which is benign, allowing for the water to be recirculated many times before toxic levels are reached.
 
The biofilter bacteria require certain specific conditions in order to be able to perform optimally, including 27°C, saturated oxygen, pH of 7.5 and darkness.  Under these conditions their performance is rapid and predictable, but away from these conditions they slow down.  Trout farmers who produce their fish at 15°C therefore require a larger biofilter because the bacteria operate slower at 15°C than at 27°C.  All systems require saturated oxygen to function at their best, and this is part of the reason why the global aquaculture industry moved to using fluidised biofilters in the early 2000’s as they provide conditions of saturated oxygen.
 
Ammonia is tested daily in most RASs to stay abreast of the trend and avoid toxic levels.  If the readings are found to approach the threshold, ensure the biofilter is clean, ensure the biofilter is aerating evenly, check that the pH is correct and if necessary add BioPond to mop up the ammonia in the water.
 
Feeding is one of the most important activities on a fish farm, but remember that over feeding can quickly result in undesirable ammonia accumulation.  As you feed your fish be sure to give them enough for good growth, but not more, to limit the risk of ammonia toxicity.