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
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
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 ˝ 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.
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