I have a 20 gallon tank with 6 red-tailed rasboras, 6 gold white cloud mountain minnows, and 3 panda corys. While feeding tonight, I noticed that one red-tailed rasbora has a weird growth behind/below his left gill area. It's reddish, and kind of big, looks like a cyst, and protrudges out laterally and down. He's gulping almost continuously, unlike the other fish which do it every now and then. Wasn't moving too much originally either, though is a bit more active now that I've taken him out of the tank and stuck him in a bowl. Don't have a quarantine tank or anything, but I don't want him infecting the other fish if it's a disease. Is this something that can be done for him, or is he a goner? Is it a tumor? The ammonia/nitrates are normal, and temps around 75.