Help!

I just discovered that one of my larger fish has what appears to be a fungus
growing on both sides of him. It is greenish in color and covers almost 50%
of his sides.. The fish is not feeding and spending most of the time near
the surface, but at a correct attitude. Does any one have any idea what the
problem could be? Should I remove him from the other fish to prevent
spreading of what ever it is? And can I treat the entire pond with anti
parasite/antifungul agents (3500 gallons, about 75 fish) without fear of
harming the other fish? Thanks in advance, Bob