Ammonia is extremely toxic and even relatively low levels pose a threat to fish health. Ammonia is produced by fish as part of normal metabolism. Additional amounts are produced from decomposing fish food, fish waste and detritus. Yet another form of introducing ammonia into a pond is doing routine maintenance such as the replacement of evaporated water. Tap water in many locales contain chloramines (a combination of chlorine and ammonia) to kill bacteria in drinking water. To find out more information on chloramines and help eliminating them, check out the Chlorine & Chloramines article.
Fish continually excrete metabolic ammonia directly into the surrounding water via special cells in the gills. In a natural environment, such as lakes and rivers, it would be immediately diluted to harmless levels. However, in a closed system such as a pond or water garden, levels can rapidly rise to dangerous levels unless it is constantly removed. Biological filtration is the most common and effective way to remove ammonia. These systems take time however to create the colonies of bacteria capable of processing the ammonia. This leaves us with the alternative of using products found in this category to control ammonia while the biological filtration is cycling (colonizing bacteria).
Raised ammonia levels affect fish health in several different ways. At low levels (< 0.1 mg/liter NH3) it acts a strong irritant, especially to the gills. The fish gill has very little protection, other than the bony cover - the operculum - and is susceptible to both physical and chemical damage. A common response of the gill to irritation - whether from parasites, suspended solids or ammonia, or any other irritant for that matter - is hyperplasia. Prolonged exposure to sub-lethal levels of ammonia can lead to skin and gill hyperplasia.
Hyperplasia is an abnormal increase in the number of cells in an organ, in this case the gill epithelium. This is similar to calluses that form on hands when subjected to physical work such as digging. Hyperplasia is usually accompanied by increased mucus production. This condition will cause the secondary gill lamellae to swell and thicken, restricting the water flow over the gill filaments. This can result in respiratory problems and stress as well as create conditions for opportunistic bacteria and parasites to proliferate. Elevated ammonia levels are a common precursor to bacterial gill disease.
Fish response to sub lethal ammonia levels are similar to those to any other form of irritation, i.e. flashing and rubbing against solid objects. Without water testing it would be very easy to wrongly conclude the fish had a parasite problem. Ammonia testing equipment and supplies can be found in Other Helpful Products.
To find products to help treat ammonia buildup, check out Pond Maintenance > Water Treatments > Water Quality.