Nitrite, water quality and fish health

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.

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.

The hidden danger

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.

(I should clarify that the colorimetric test kits used by most hobbyists  measure nitrite-nitrogen. This is just the N bit of NO2. To convert nitrite-nitrogen to nitrite simply multiply by 3.3)

The danger signs

The signs of nitrite toxicity are similar to many other fish health problems, which is why it is so important to carry out a full investigation and diagnosis before any treatments are used. At low levels the fish may rub against solid objects or ‘flash’. As levels increase the fish will become lethargic, but may still swim up to feed. If the fish is suffering from nitrite poisoning the gills will be pale tan to dark brown in colour and the fish may exhibit signs of respiratory distress, i.e gasping at the water surface or hanging around water inlets. Moving fish to clean water will often reverse the condition.

How to control elevated levels

In the next stage of the nitrogen cycle  Nitrobacter sp. bacteria oxidise nitrite (NO2-) to nitrate (NO3-). Nitrate is considerably less toxic than nitrite, although nitrate levels should not exceed 50mg/litre.

The situation with nitrite is similar to that of ammonia, inasmuch that Nitrobacter bacteria will only be encouraged to multiply if there is an increased nitrite level. Unfortunately, all nitrifying bacteria are slow growers and Nitrobacter take some time – often 2-3 weeks in new systems– to respond to an increased level. In the meanwhile the water has to be managed by regular testing and water changing until  levels stabilise. Nitrobacter can be inhibited by very high ammonia levels and prefer a pH range between 7.5 – 8.6 with at least 1mg/litre dissolved oxygen.

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.

Other sources

In general we would only expect to see elevated nitrite levels during the maturation of new set-ups as part of the normal filter conditioning process. However, poorly maintained or overstocked systems may also suffer from raised levels or occasional  ‘spikes’. The other situation we should be aware of is that under hypoxic conditions (low dissolved oxygen levels) many bacteria can reduce nitrate back to nitrite.