Fish loses an eye?

you should move the fish to a hospital tank, that is to many fish for this size rainbows are territorial. Maybe get a ten gal and just move the tetras there and keep them there. melafix should be used for 7 days then change the water, always move a sick fish you never know when it can spread. diseases in fish are almost always caused by stress, you have to find the stress and eliminate it. in the ocean if fish are being bothered they can swim off and not return, not the case in a tank, they are stuck together, I am guessing this is part of the problem.