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About Ammonia, Nitrite & Nitrate (The Nitrate Cycle)




Ammonia is the first stage of the nitrogen cycle with the majority of ammonia being produced from fish waste. Ammonia is extremely toxic to fish and will quickly lead to fish deaths when present in the aquarium water. Although ammonia is toxic, ammonium is non-toxic. Ammonia & ammonium are easily converted into each other, with the two being dependant on the pH of the water. If the pH is acidic then ammonium will be present instead of ammonia and this is harmless to fish. As the pH is a logarithmic function, so is its control over the presence of ammonia. For example, if ammonia is present in the water and the pH increased from 7.0 to 8.0, there will also be a ten- fold increase in toxic ammonia which could lead to the loss of all the fish in the aquarium.


Therefore, it is very important to know that ammonia levels are zero before ever increasing the pH of an aquarium. Many manufacturers produce ammonia test kits in both liquid and tablet form - all being easy to use. As a rule, calculated ammonia levels above 0.5 ppm are considered dangerous & may lead to stress or death. Visual signs of ammonia toxicity include: Fish gasping at the surface of the water, cloudy eyes, and frayed fins. Ammonia may cause kidney damage, affecting the balance of the fish, which may result in dropsy. High levels of ammonia can also result in gill tissue damage which reduces the oxygen- carrying capacity of the blood, this can lead to death by suffocation at a much later time, even after the ammonia levels have become safe.


The main reason for ammonia in the aquarium water is poor filtration or over stocking. Using the correct size filter for the amount of aquarium water is essential in allowing bacteria to grow and multiply to the required amount. Insufficient amounts of bacteria will lead to ammonia poisoning. Always make sure your filter is working to its full potential, that the filter media is replaced as per the manufacturers instructions and that the filter foams are only washed out in aquarium water (tap water contains chlorine which will kill the essential bacteria).


As smaller fish have a higher gill surface area relative to larger fish, they are more prone to ammonia toxicity problems. It is also very important to note that in the absence of oxygen, nitrites are converted back into ammonia, therefore, it is important to ensure that filters do not become blocked, or switched off for any length of time. Water changes are the best way to quickly solve ammonia problems. Frequent partial water changes, performed over a period of 4 or 5 days will reduce the ammonia levels. Also, decreasing the pH to levels to below 7.0 will decrease the toxicity by converting ammonia to nontoxic ammonium. The use of ammonia absorbers can be helpful in removing ammonia, but do not use these absorbers during a maturing period. Aquarium Pharmaceuticals manufacture "Ammo Lock 2" which helps fish overcome high levels of ammonia. Other manufacturers also provide products which will help eliminate ammonia.




Nitrite is produced from ammonia by nitrosomonas bacteria and is part of the Nitrate Cycle (Ammonia, Nitrite and Nitrate). Nitrite poisoning is also known as 'brown blood disease' because the blood of the fish turns brown due to an increase of methemoglobin which renders the blood unable to carry oxygen. The fish literally suffocate even though there is sufficient oxygen in the water. Levels above 1.0 ppm for freshwater are considered unsafe.


Here are some ways to solve nitrite problems: As with ammonia, the quickest way to remove nitrites is with water changes. For high levels, 50% water changes must be conducted daily for about 4 days, this will help to dilute nitrites to safe levels. The addition of aquarium salt will help prevent methemoglobin from building up. Aeration should be increased to provide ample oxygen saturation in the water. Feedings should be reduced and no new fish added.




Nitrite is converted to NITRATE by bacteria in the filter system (nitrobacter bacteria). Nitrate (NO3-) is the end production of the nitrogen cycle & is relatively nontoxic. The level of nitrates in the aquarium slowly rise over a period of time, with high levels becoming toxic to fish. Deaths among newly introduced fish are usually caused by high nitrate levels in the aquarium.


Plants & algae utilize nitrate as a food source, which will reduce the levels. In freshwater aquariums, the appearance of nitrates suggests that the biological filtration system is operating. It is recommended that nitrates be kept under 60 ppm. Partial water changes or nitrate removing sponges/resins are the best method of keeping nitrates under control. 



Urmston Aquatics