OK, the parasite definetly is the visible part of ich, but it is almost always there. How come just one fish will have it? The new fish or a small fish, or a certain kind of fish? It is "BROUGHT ON" by bad water #1 reason, usually high nitrates or ammonia, #2 water getting cold too fast, or staying too cold, #3 reason, fish fighting or just getting bullied, or even a bigger fish scareing one. Stress is what brings it, to cause the fish to actually get ich. If he checks his water parameters, temps, and at this point only has one fish, but with a waspfish and a leopard wrasse he most likely had a predator tank, which of course there is a lot of aggression and space invasion. The 2 times I had it in my tank, was as I said when I forgot to turn back on a tank heater after a water change. I had about 6-7 fish in the tank, but both times it was a female gold banded maroon clown that had it, and all I did was turn the heater on, and it went away, and never came back.