Nitrities as no good and very bad for fish. You shouldn't have any in your tank as once your tank is cycled as all ammonia and nitrities get coverted to nitrates, which aren't as bad for a fish only tank.
In fish only tank the fish can stand high nitrates, except some fish where it's a problem. High nitrates can cause slow growth and some stress in fish. Most fish once they're in the tank get use to the high nitrates and are usally fine with it. Problems that can arise is when adding a new fish that is not use to the high nitrates and this can lead to problem and even death.