Nitrite in a pond is a by-product of biological filtration.Although it is less toxic than ammonia, nitrite can still become problematic if levels begin to rise. Nitrite can impair fish’s ability to absorb oxygen from the water. Fish affected by nitrite poisoning will show signs of suffocation – gasping at the surface, etc.As with ammonia, nitrite is measured in mg/l or ppm. It is recommended that nitrite levels should not exceed 0.2 mg/l. Higher recordings can be reduced by improving the biological filtration, and reducing stocking and feeding. High nitrite levels should be diluted with a partial water change.