Sanding the tank ceiling is not going to help. It's hard to tell in the pic but I would think this issue is being created by either a sagging ceiling or a warped rhp. My guess, it is the ceiling that is sagging and to fix the issue you need to beef up the ceiling and give it better support. Maybe something like a small piece of flat metal bar cut to the length of the depth of the cage would allow you to snug the rhp down and pull the ceiling back into place.

Or, maybe a good diameter pvc pipe cut to the excact height of the cage and wedged inside to lift the ceiling would work better.