KIA111,
That is probably the tang turbellarian disease. I've seen it infect tangs and some butterflyfish, but never seen it affect damsels like your chromis. As mentioned, quarantine is the way to go - but in your case, you might consider just waiting it out. Many time, this disease is self-limiting, it goes away on its own after a few weeks to months. I don't recall every losing a fish to it. Many of the "treatments" are really uncontrolled - so if you treat it and it goes away, did the treatment really work? Or was it just the self-limiting issue taking place?
When you say the fish has it "all over" how many spots are you talking about? 10 on one side? 50 on a side? More? Also - since you didn't quarantine, be aware that you may see other diseases overlying the turbellarian problem. For example, a tang with turbellarians that also develops gill flukes is going to go downhill and die with the only visible symptoms being the black spots and slightly more rapid breathing - very easy to assume the turbellarians killed the fish, but not accurate.
JHemdal
