While he was in a quarantine tank, because of some aggression I've had to put him in my full 75g tank, so I'm worried as to what this ... growth? on the side of my betta is. He's had it since we got him, so it's not something that my tank caused.
I wonder if it's ... scar tissue or something? When we got him, someone had been keeping him in a bowl (one of those tiny wal-mart bowls) with another betta. He was almost dead, or so it seemed. (The other betta is the one we actually had to move from the 75g because of aggression towards the other fish.) After giving him some wide range medication (supposed to treat for fungus types, because we weren't sure what it was) he seemed to perk up...
After putting him in the 75g, he's brightened colour considerably, and is very active. He eats healthily and, apart from the occasional "poof" at the guppies, he gets along fine with the other fish. If anything, being in the big tank has made him healthier.
Just the blue growth or whatever it is (it's not fuzzy, and it hasn't grown any in the time we've had him) is still there and I don't want it to be something that infects the 75g. I just didn't have a choice in moving him.