Ok, before he started having fry die off problems some time ago he was using Novaqua, which neuteralizes chlorine, splits the ammonia/chlorine bond of chloramine, leaving ammonia. Around here they tend to jack up the additives during foul weather, this is more common during winter months. This was leaving him with an ammonia increase after water changes that his bio filtration could not handle. Most water donditioners that do not neuteralize ammonia contain nothing but sodium thiosulfate, notice the sulfa part of that name.
He swithched to Prime for a while, the fish did much better, but he was having this rash problem when working on his tanks. Prime also contains sodium Hydroxymethane sulfinate, another sulfa compound. Apparently this is what was causing the rash.
He is now using nothing but sodium thiosulfate, mixed from crystals, and doing smaller, more frequent water changes, and doing just great, with no rash problem.
Here is a list of some common water treatments, what they do, and an overview of what they contain;
http
/www.thetropicaltank.co.uk/rev-cond.htm
Sulfa drugs are older antibiotics, previous to your various penicillin based drugs. This is something you may want to mention to your doctor if you switch water treatments and it does cause the rash to go away for you, as a reaction to sulfa based drugs could cause you to have a serious reaction if the are prescribed for you.
