ammonia and nitrite are fairly dangerous, to the point of being deadly . . . other than in a tank that is cycling, you would ideally keep ammonia and nitrite at 0ppm
nitrate is more arguable, some would say keep it below 40ppm, others 20ppm, etc., . . personally, I feel best when they are at 10ppm or less . . .
nitrate is signifiicantly less toxic to fish than ammonia/nitrite, but the common wisdom is that prolonged elevated levels can lead to fish discomfort, susceptibility to disease, and perhaps even death as an indirect result
note that some fish are more nitrate-sensitive than others, e.g., Discus and Rams are good examples . . .