Agree on that one. Could be a birth defect or a wound that happened while he was growing and didn't heal properly. I have an Argentea with this issue, I got her this way and it doesn't seem to cause any problem.Judging by the shape of that split, and if it came to you that way, I'm thinking maybe a minor cosmetic defect rather than an injury to the dorsal. Happens sometimes, in various species you can get a few like that as they grow out from fry-- what a breeder does from there depends on the breeder, the species, the overall look of the fish, etc.