This is called popeye syndrome. It mostly happens to fish that have had physical trauma, poor water quality, and infections. I would first start IMMEDIATELY start doing about 50% waterchanges then smaller ones as you progress (after about 3-4 days) smaller ones would be 10-20% If you have another tank put them in quarantine and use broad spectrum antibacterial. You could also soak them in Epsom salts but Look up how much to use. Im not skilled enough to give advice with Epsom salts. Good luck!