What does NO2 in the aquarium mean in practice?
Nitrite occurs between the oxidation of ammonia and nitrate, especially in an immature or overloaded biological filter.
When does it matter?
They interfere with oxygen transport in fish and in a stable aquarium should remain at a level undetectable by an appropriate test.
Which solutions are related to it?
If the result is positive, feeding is limited, aeration is increased, water changes are performed and the biological bed is protected against chlorine.