Ammonia is formed from the metabolism of protein and is the major waste product of fish. The majority of ammonia from fish is excreted through the gills, with relatively little being lost through urine and feces. Ammonia is also formed as uneaten feed or other organic matter in an aquarium decomposes. High concentrations of ammonia in the water make it difficult for fish to eliminate ammonia from their bodies. This buildup of ammonia can cause stress, gill and internal organ damage, and eventually death.
Total ammonia is comprised of two components: un-ionized ammonia (NH3) and ionized ammonia (NH4+). Un-ionized ammonia is extremely toxic to fish whereas ionized ammonia is not. The proportion of un-ionized to ionized ammonia shifts in relation to pH and water temperature. As pH or temperature increases, more of the ammonia shifts to the un-ionized, toxic form. Un-ionized ammonia begins causing gill damage at approximately 0.05 mg/L and death at approximately 2.0 mg/L. Keep in mind that most test kits measure total ammonia and not un-ionized ammonia but may make no reference to the difference. A table is used to calculate the portion of un-ionized ammonia from total ammonia.
Ammonia is removed from an aquarium system through the use of a biofilter. The biofilter provides a substrate on which nitrifying bacteria grow. These nitrifying bacteria consume ammonia and produce nitrite, which is also toxic to fish. Other nitrifying bacteria in the biofilter consume nitrite and produce nitrate. Nitrate is not toxic to most freshwater fish, except in high levels, and can be reduced sufficiently through periodic water changes. Some nitrate will also be utilized by plants. In natural systems and some aquarium systems, nitrate is converted to nitrogen gas by denitrifying bacteria. This conversion from ammonia to nitrite to nitrate to nitrogen gas is known as the nitrogen cycle.
Nitrifying bacteria, although naturally present in the environment, will take time to accumulate on biofilter media before they can efficiently remove all of the wastes produced by the fish. Setting up a tank and stocking it full of fish the same day, or even within a week or two, is a sure recipe for disaster. The bacteria in the biofilter can require three to eight weeks to cycle (i.e., become established) at 77-80 degrees F; even more time may be required at cooler temperatures. Aquarists can begin this cycling process by utilizing one or more of the following methods:
As a biofilter cycles, ammonia will rise until sufficient nitrifying bacteria are present to consume the ammonia and convert it to nitrite. Ammonia levels will then begin to decrease while nitrite levels increase. Nitrite levels will continue to increase until sufficient bacteria are present to consume the nitrite and convert it to nitrate. Unless many plants are present, nitrate levels will rise slowly until a water change is performed. Ammonia and nitrite tests should be conducted every few days until the ammonia level and then nitrite level spike and lower (i.e., cycle). Tanks that are stocked before the biofilter is functioning properly will often become cloudy because of heavy non-nitrifying bacterial blooms.
After the cycling period, ammonia should be tested once per week. Once established, a well-managed aquarium should have no detectable levels of ammonia. If ammonia is present, management of the aquarium should be evaluated to ensure:
Nitrite is formed by the conversion of ammonia by nitrifying bacteria. Nitrite is toxic to fish because it binds with the hemoglobin in fish's blood to form methemoglobin. Hemoglobin carries oxygen through the body while methemoglobin does not, so fish in high nitrite waters may suffocate even if sufficient oxygen is present. If enough methemoglobin is present in a fish's blood, it will cause the blood to appear brown instead of red. Consequently, nitrite toxicity is often called "brown blood disease."
If nitrite is present, a water change should be performed. If the fish species in the tank will tolerate increased salinity, salt can be added to the aquarium to increase the level of chloride in the water. Chloride levels should be at least six times greater than nitrite levels. Chloride prevents the uptake of nitrite by the fish's blood thus eliminating the effects of brown blood disease. However, the addition of salt/chloride does not remove nitrite from the system, and the source of the nitrite must be controlled.
After the cycling period, nitrite should be tested once per week. Once established, a well-managed aquarium should have no detectable levels of nitrite. If nitrite is present, management of the aquarium should be evaluated to ensure:
Nitrate is formed from the breakdown of nitrite by nitrifying bacteria. Except in very high concentrations, nitrate is not toxic to most freshwater fish. Nitrate can be absorbed by plants or removed from the water through periodic water changes. In natural systems and some aquarium systems, nitrate is converted to nitrogen gas by denitrifying bacteria.
Nitrogen gas can be produced in the aquarium from the conversion of nitrate. However, it is often of more concern in some underground source waters, particularly water from wells and springs, where nitrogen may be at supersaturated levels. As with oxygen and carbon dioxide, the solubility of nitrogen increases with decreasing temperature and salinity and increasing pressure. The pressure reduces as water from a well or spring reaches the surface and less nitrogen gas can be held. If this water is put directly on fish, bubbles can form in the gills, skin and fins as the nitrogen begins to vent off in an attempt to equalize with the nitrogen in the atmosphere. This problem, known as Gas Bubble Disease, is similar to the bends in divers and can cause death if the condition is severe. Well or spring water should be degassed before coming into contact with fish. Vigorously aerate the water with an air pump and airstone for approximately 24 hours before the water is added to the tank to ensure excess nitrogen gas has been removed.