Nitrite is produced continually during the ammonia cycle by nitrifying bacteria in the biological filter. While not as immediately toxic as ammonia, any accumulation of nitrite should still be addressed as soon as possible. The nitrite level will eventually be broken down to non-toxic nitrate. Regular testing for nitrite is recommended - chronic exposure to even low levels of nitrite can cause brown blood disease in fish, which can cause suffocation and death.
With the exception of a system establishing a biological filter and progressing through the nitrogen cycle, the nitrite level in any tank should ideally be zero.
