Have any apple snails, plecos, or Fei Fengs in there? Looks like a wound to me, either burn, bites, or abrasions. I've had rays that like to surf the bubbles up to the top of the tank rub their back on things and it ends up looking like that...... Ray equivelant of a scab.
My tank size is 12' 6' 2'. Yeah I think is my fei fong. I already took that ray out and she is still eating hope nth goes wrong.
Sent from my iPhone using MonsterAquariaNetwork app