Swimmer's itch comes from cercaria- worm larvae that pass part of their life cycles in Nassarius/Ilyanassa snails...the other part is spent in water birds like ducks and geese. I have had swimmer's itch many times...always from fishing in a brackish tidal creek, on an outgoing tide, after rains have lowered salinity even more than usual. Its not a rash...its individual red raised spots, like an insect bite, except that the itching is much more intense and steady. The spots fade away and stop itching after about 4 days. Discoloration may remain for weeks. Reef salinity is too high for swimmer's itch. While I have never gotten swimmer's itch from my tank, despite all my Ilyanassa snails, I have gotten rashes. Mild ones come from scraping the salty edges..they go away fast. However, I did once get a very itchy, persistent rash that looked like poison ivy or some other kind of contact dermatitis. I went to my dermatologist, who took a scraping and prescribed bactroban cream, an antibiotic. It disappeared within hours of starting the cream, so it had to have actually been some form of bacterial infection.