There are also bacteria that can do that but eye have never seen it without the water having either ammonia or chlorine problems, I have seen it after a friend did a 50% water change and forgot to dechlor. a couple hours later he saw the fish acting up and realized what he had done and dechlored it then, a few days later a couple of the fish had a milky layer over their eyes. I don't remember what he used to clear it up.