I picked up a used 45 gal tank and stand. It's pretty decent but on the outside where they had a kind of holographic background there is this bad stain, for lack of a better word. I think the background had gotten wet and some nasty stuff is stuck on the back of the tank. Vinegar and a razor blade doesn't touch this stuff. When I get it wet it looks like it disappears but as soon as it dries it's back uglier than ever! Any ideas on how to get rid of it?