HELP! I have a 180 gallon fish tank. I recently bought 2 motor rays which are my first rays. They have been in the tank and they were doing great. This morning I realized one had cuts on his outer disk. What is this from? How can I stop it?. My tank is completely sand with a couple of round rocks. The fish are Payara, Arowana, Largemouth Bass, Indo Pacific Tarpon, Pictus cat, and the 2 new rays (all 5-7").