I have learned quite a bit about the cycling process, but I have never read anything that mentions at what level nitrite begins to be harmful to fish. I have learned that high levels are quickly harmful and I understand that a reading of anything above 0 will affect fish in a negative way over an extendend period of time. At what level does nitrite is start to become a problem, 0.5ppm, 1.0ppm, 2.0ppm, or higher? The reason I ask is because am currently in the middle of my tank cycling and nitrite has been reading at 1.0ppm for two weeks now. It has dropped from 2.0ppm, but seems to be stuck at 1.0ppm. This level does not appear to be negatively affect my single goldfish that is in there.