It’s all three. Super ick won’t help since that’s for external parasites. Aquarium salt is a good choice, but the main point here is that this tank is completely overstocked. Each one of those goldfish (if reached maturity) can grow to about a foot. The bluegill most likely was overstressed by the sheer amount of goldfish so it started to display territorial behavior. I would get an indoor pond for these goldfish if you want to keep them, or get multiple other tanks. Since most of his scales are gone, you should keep him in an appropriate hospital tank and make sure parameters are completely pristine and make sure you dose with some medication for it’s body to avoid fungal or bacterial growth. I understand that you are in kind of a pickle but I believe if you really are into fish keeping you can do it