Black Spot Problem

[Jade06]

I recently bought 3 chiclids to put in my 30 gallon tank. I noticed when I bought one that it had a black spot by its fin. It was small and I thought nothing of it. Now...3 months later the black spot has spread and it is on its fin and down its sides. It looks speckled. It seems to be doing fine(eating ok, not rubbibg itself on anything, not breathing hard). None of the other fish have these spots, just the one. Is this a fungus or parasite? What treatments do you recommend.