This is nitrite, not nitrate? Just checking, it's easy to get one letter wrong.
Secondly, is it 2 to 5 mg/l or something else? I suppose it can't be mg/l, for if there really was 2mg/l nitrite in the tank, your fish would be dead or dying by now.
If it's nitrite, then it's bad to have any at all in the water, because it's poisonous to fish. The nitrite level should be at 0 at all times. If it's not, there could be something wrong with the beneficial bacteria. Is the tank cycled?