I just noticed these marks on my 15in freshwater female stingray she didn't have them yesterday that I've seen. The heater in the tank has a guard on it and its nothing in the tank but sand and a few smooth rocks she shares the tank with a 7in male stingray and 8in arowana
Check ur water parameters. Could be from ammonia. Clean water and some salt should heal it Happy to say she's all healed up ?I checked my water parameters & the water was fine she loves to move her rocks around if it wasn't from that it had to be playing with heater I'm guessing even with the heater guard on she still found a way to burn herself I moved it to the sump sump