We have 4 goldfish in are tank that are now rather large. One of them started swimming upside down and sideways a few days ago. We separated him and fed him a couple peas (which was suggested for swimbladder). He started swimming upright again yesterday, but now has a very noticeable red bump on his side. He seems to be getting better other than the bump and still a little off balance. The other three goldfish and two plecostomus seem healthy.

It isn't a sore, is it? I ask because plecos are known to go after fish as they rest at the bottom in the evening, as plecos are nocturnal.
I have a pond filled with gold fish, some have had bumps for years. Not pretty, but not dead either. Outdoors it could come from sunburn, inside maybe illness. I believe I read once it is also cancer. If they eat and swim uprite they should be OK.

Well, the fish is still alive - surprisingly, but he lays on the rocks on his side most of the time. When he does swim, he is sideways and bumps into the other fish and the glass. He has noticeable bumps on both sides of his body. It's been a few weeks now, so he must be able to eat some of the food, but he just keeps getting worse. I do wonder if it might be cancer or a tumor of some sort.