Hi Chick-in, how is Hank doing? Were you ever able to move him out of the hospital tank?
I have some black neons in quarantine right now and noticed the same thing with one of mine. I initially treated with Maracyn because I thought it was some kind of infection but looking closer I think it is just missing a gill plate after all. From what I’ve read it could be a birth defect, the fish eats and acts completely normal. Just proves how hardy these little fish can be.