There are no tank makes except for an arowana half it's size who has only been with him for a few days during a tank transfer and they leave each other alone. Also the water is kept very clean. Whenever I change the water I always add salt with prime. That's why it is so odd this happened to him and I can't figure it out.
There is just a small amount of gravel on the bottom to cover the undergravel filter and a smallish rock he likes to perch on. It could be possible that the heater could have burned him since I can't think of anything else. How do go about treating a heater burn? =)