Nitrite, Nitrate Poisoning. Nitrite/Nitrate Poisoning

Another serious affliction that results in fish casualties, nitrite poisoning is similar to ammonia poisoning in many ways.

How it occurs·
  • Nitrite (NO2-) is formed when Nitrosomonas sp. bacteria oxidise ammonia produced by fish and decomposing organic matter. Although it is less toxic than ammonia, elevated levels still present a threat to fish health. Prolonged exposure to low levels can lead to stress and is often associated with stress-related disease such as bacterial ulcers and fin-rot.
  • As a result, the colours of the fish become more intense or really bright, panting, and hanging just below the water surface. At higher levels, skin and gill epithelia can be damaged and opportunistic bacteria and parasites may take advantage of stressed fish. The main danger from high levels comes about because it is actively transported across the gills and into the fish’s bloodstream where it oxidises normal haemoglobin to methemoglobin.
  • Normal haemoglobin picks up oxygen at the gills and transports it to the body tissues where it is exchanged for carbon dioxide. Methemoglobin cannot transport oxygen and therefore in acute cases the fish will be effectively asphyxiated.

What to do
  • It can be tested by a Nitrite/Nitrate water test kit. Nitrite toxicity varies with fish species depending on how much is taken up and how effective they are at converting methemoglobin back to haemoglobin, but as a rule of thumb nitrite levels over 0.1 mg/l should be seen as unacceptable. This refers to nitrite-nitrogen as measured by most hobbyist test kits.
  • The best course of action, is to reduce feeding, make frequent partial water changes until the compounds are reduced, and increase the aeration in the water.
  • Understanding the causes of nitrite poisoning is important in order to learn how to avoid it. Heavy organic load or pollution and not doing enough water changes are largely responsible for nitrite poisoning. Also check water supply, some are high in Nitrates to begin with.
  • Nitrite / Nitrate poisoning is not a disease but will kill your tropical fish if not attended to. It results from having a large bio-load on the filtration system or from not performing enough water changes. Perform a partial water change immediately and monitor the nitrite and nitrate levels closely until the situation is resolved. You may have too many fish in the tank and will need to perform more frequent water changes. Nitrite readings on your aquarium water test kit would indicate that your tank is still in the aquarium nitrogen cycle nitrite phase, or it is undergoing a mini-cycle if you've recently added more fish to the tank. It is worth noting that the actual toxicity of nitrite is affected by salinity. In salty water, that is water with added sodium chloride, the gills will tend to take up chloride ions in preference to nitrite ions, thus offering some protection against nitrite poisoning. Adding ½ oz of salt per gallon, along with regular water changes should suffice as a short-term measure against poisoning. Aeration should also be increased.In the long-term the aim is to maintain non-detectable levels at all times. If levels rise above 0.1 mg/litre immediate steps should be taken to remedy the situation.