fish has a lump on one side

Hi everyone,
My gold fish "Spanky" has developed a lump on his right side. He seems to be swimming fine and eating. I have no idea what this is and don't know what I should do. Can anyone give me suggestions as to what this may be and how I should treat him? Please let me know.
Thanks again!!

Unfortunatly I can't send a photo. Do you have an idea of what it may be? His side isn't bloated like in dropsy. His side is swolen from the middle of his side to his gills. Sure hopw someone has an idea of what it is. Hope it isn't serious. Should I isolate him?

I could never flush him. He's swimming and eating. Is there no way of treating this? Have you seen this before?

Thanks for the advice. I understand what your saying about the flushing comment, but I tend to get way too attached to my fish. I would hate to see him suffer, but other than the lump he's still swimming and eating fine. We tried medicating the tank, I hope that works, and I hope the other fish don't get sick too. I think I will call the pet store and see what they say. I have a feeling your right and he probably won't make it, but right now I just want to try and see if I can do anything. As of last night he ate like a little pig and was swimming around with the other fish. But I did catch him laying on the bottom of the tank. As soon as I turned on the light he was back swimming with the other fish. I'll keep you updated and let you know if any of the meds works.
Thanks again!!

spanky update

Well, poor spanky is not doing so well. Everyone was right. I was told there was no hope and we have tried everything and his lump is getting bigger and he's spending more time at the bottom of the tank. He is still eating and when he sees us he starts swimming around w/the other fish. But I can tell he's getting worse. Any other ideas on what I should do?? I would hate to see him suffer. Please let me know.