If it's the snake head it will usually be bites at the edges too. If you have a slum tank or some overhead or IOS, put the heater there to avoid such burns.
like 5inch or 6inch's, compared to the ray @ 9-10inch's ..
but when i think about it, the ray likes to hang out in the current near the outflows & guess where the snakehead likes to chill, behind the outflows.. lol so maybe its a territorial issue going on.