Oh ok, new new. Maybe heater burn if it just happened. Id watch the fish, but it looks like a large area on it. If this is infact recently from the heater you may want to consider treatment for infection especially if tissue sloughs off or it becomes open. As to treatment I don't know and would have to read up on this, plus you have prazipro in the tank. Hopefully some one who has dealt with this will chime in.