We have had a new 48L tank running, with no fish in it for three weeks and have high nitrite levels. I have done 20%, 50% and completely changed the water, as advised, without any variation in the nitrite level. I have tested the tap water we are putting in , which is fine. We have tried filter start and cycle treatments but again it makes no difference. We have never had a fish tank before and are completely stuck as what to try next. Please help!!