I bought two platys Saturday and put them in QT. 5 gallon cycled ammonia 0, nitrite 0, nitrate < 10, temp 78f. Last night I noticed one has lost scales on his back and looks like he's been cut with a knife. Its white but does not look like cotton, just more like an injury. It is worse today. I dosed with Melafix. He's eating fine but it looks so painful. His tank mate is fine so far. Any ideas as to what is going on?