Just to put my two-penny worth in lol, the key sentence in that article for me is "Fish are typically able to survive in seawater with more than 100 ppm nitrite!", and the key word is 'survive'. I'm sure fish are able to survive in water with nitrite in, hence people successfully using Damsels to cycle tanks for many years. However, my aim is for my fish to thrive, not just survive and any level of toxin in the water will cause the fish undue stress and reduce their ability to resist other diseases/infections. Therefore, nitrites may not kill fish outright but they will certainly give the fish less chance of resisiting other diseases that may well kill it. This also applys to nitrates, which again are not deadly but will cause undue stress to the fish at high levels. I agree with the point that in an established tank, there should not be any ammonia or nitrites and therefore these don't need to be regularly tested for. In this situation however, the presence of ammonia or nitrites indicates that the bio-load of the tank is exceeding the ability of the bacteria to process it. This should rectify itself with time and good maintenance.
Matt.
