That's a great help, thank you so much
Do you use the API ammonia test, by any chance? That often looks like it's showing a slight positive when it is zero. Anyway, I can't see anything in any of your stats that give much cause for concern and if all the other fish are okay, then we can probably rule out environmental problems, although I do have to register concern over the crab; they don't really mix with fish and a prone to attacking fish when they're sleeping.
If the scales are sticking out it's probably dropsy. Unfortunately, dropsy's a symptom, rather than a disease in itself, but it's normally associated with organ failure and the fish are rarely curable, I'm afraid to say.
You could try a general medication (I've always had good results with esha 2000), but I'd certainly treat the fish in isolation (if you can catch it in a 1200l !); you don't want to risk the rest of your stock.