I return with more issues.
My black ranchu has a lump at the top of his pectoral fin, where it meets the body. It's a perfectly round, black, lump, no discolouration at all. I tried to get a picture, but since he's matt black, and the lump looks to be under his skin and matt black as well, it's not showing up very well, especially since he's extremely wiggly :/
He's swimming normally as far as I can make out, but it is pushing his gill out a tiny amount when he swims.
I did recently add another fish to the tank who was a bit of a biter for his first few days, but he's settled down now.