Epsom salt would relax the muscles and reduces swelling, a more effective treatment for constipation than for healing.
But, still I would not use table salt for the OP's purposes (some may disagree with me).
I would use melafix, although treating that much water may still be expensive.
To the OP, what do you think of using melafix? Or is there the possibility that you just have incompatible fish? Maybe you could re-home some of them?
You have big tanks, so I am sure they are big fish (not easy to rehome), How about building another monster tank and separating the fish. Ok, that suggestion may not help you, but I did just give you an excuse to get another tank!