When nitrites are present at any level in the pond, you need to take it seriously. Nitrites can bind up the red blood cells in fish, causing brown blood disease, which can be fatal. One sign of high nitrites is the fish gasping at the pond surface.
What can cause high nitrites?
All the things mentioned above for high ammonia levels could also cause high nitrites. One note is that the Nitrobacter bacteria, which removes nitrites, is much more fragile than the Nitrosomonas bacteria, which removes ammonia. This nitrite reducing bacteria takes twice as long to develop in a new filter and is the first bacteria to die off if you run a strong medication through the filter media. Also, if a filter gets plugged up or isn’t cleaned often enough, nitrites will build up in the pond even before the ammonia does.
How do I lower high nitrites?
Just like with ammonia, you want to perform water changes, stop feeding the fish, and add oxygen if necessary.