Try this, Vince:
Shine a flashlight or something through the front of the tank, so that when he comes cruising by, the light will shine off of his scales and let me count the scales between the LL and the pelvic fin. It's gonna be the only way that I can tell for sure.
He's a fairly large fish and fairly thick, which right off the bat makes me think brevis, but he simply doesn't LOOK like a brevis to me. The head is to small. I hesitate to make a firm decision on this, given the circumstances. Especially Co sidering the fact that we've seen pretty much NO large FATFs in the hobby, I'm not really sure what one would look like in-tank.