My foot long gold fish has a large swollen belly, with what seems to resemble a large red blister?

The fish is about 8 years old. I have 2 gold fish (they look like koi but are not) about a foot long, and 1 black sucker like fish in a 45 gallon tank. Yesturday evening I came home to one of the fish whose belly started to swell about 3 months ago making it look pregnant but it wasn't and now the fish is lying at the bottom of the tank with a large tumur like bubble on it ...I don't know what to do, who to call...the other fish is trying to comfort it and is getting stressed out. Any suggestions?