It isn't quite that simple. The critical level of ammonia is going to be based on your tank's temperature, pH, and other factors like what fish you have in there. I've put together an article with guidelines on when ammonia levels hit critical concentrations (based on temp and pH): [URL="http
/www.fishforums.net/index.php?showtopic=154313"]http/www.fishforums.net/index.php?showtopic=154313[/URL]It isn't quite as simple with nitrite, because unlike ammonia which has an ammonia-ammonium balance that can significantly shift how dangerous the pollution level really is, the nitrite-nitous acid balance is almost always fully shifted toward nitrite, so the splits just aren't as important. Basically, when the nitrite starts getting over 1.0 ppm, you should be considering doing a large water change. Again, this specific number is going to be pH, temperature, and species dependent, but I think that 1.0 is a good guideline and an easy-to-remember number.
