Nitrites (NO2)

Nitrites are the intermediate product of the nitrogen cycle. Like ammonia they must be at zero in a mature aquarium.

Detailed explanation

Nitrosomonas bacteria transform ammonia into nitrites. Then the Nitrobacter (or Nitrospira) bacteria convert the nitrites into nitrates. In a well-cycled aquarium this process is continuous and nitrites remain at zero.

The problem with nitrites is that they bind to fish haemoglobin instead of oxygen, forming methaemoglobin. Fish suffocate even if the water is well oxygenated. This is why nitrite is called the silent killer: fish don't show obvious symptoms until it's too late.

There is a little-known peculiarity. Chloride ions present in salt water reduce the absorption of nitrites through the gills. For this reason, at the same concentration, the toxicity of nitrites for fish is generally lower in marine aquariums than in freshwater aquariums. But that's no excuse to tolerate detectable nitrites.

In ponds the nitrite problem often occurs when the filter is restarted in spring. Even adding commercial bacteria serves 2-3 weeks for the Nitrobacter colony to stabilise.

Reference values by aquarium type

How it is measured

Drop test (API produces a reliable kit for freshwater aquariums). For Salifert marine aquariums. The colour change towards purple/fuchsia indicates the presence of nitrites.

What happens when it is too low

No problem. They have to stay at zero.

What happens when it is too high

Fish breathe heavily on the surface. The gills turn brown (methaemoglobinaemia). Rapid mortality even at 0.5 ppm in freshwater aquariums. Adding table salt (NaCl) as an emergency measure may temporarily reduce toxicity in freshwater aquariums.

Related terms