the nitrate level is 40 ppm. Currently there are 12 fish, but the problem existed before that many were there. There are no skeletons. The tap water has no nitrate. The fish are an arowana, 2 gars, baracuda, 2 tinfoil barbs, 2 cats, a Severum, 2 geophagi, 2 green terrors. They appear ok now...