I've been away for 3 days and came home to this. It wasn't there when I left. It can be felt with a fingertip/ fingernail on the outside of the tank, but not at all on the inside. 75gallon.
I do have a super curious cat who has been up around the tank lately. I had blocked her usual access point while we were away, so I'm thinking she may have tried to launch herself in that direction and missed and scratched the outside. Or maybe I'm grasping at straws and it's a crack.
Tank is flush on all 4 corners and level, but there's a small space between the tank and stand in the middle, about 6". Less than 1/8", Marineland said not to worry about it, but of course now I am.