might be tumors; I've had some gf with that. Unfortunately, most have died but it took a few seasons. They acted just fine and one day, I'd find one or two either gone or dead. I'm pretty sure they get these when the water conditions are not great. Most of the ones I saw happened before I expanded. Like you said, not much you can do, but do test the water maybe a bit more often just to be sure everything is hunkie dorie. If I had to guess, I saw more of this in early summer/late spring (when I first noticed, that is) though more often when they died, it was from coming out of winter, so maybe it affects them harder, at that time. Not an expert re tumors and fish...