Ok, maybe I don't have my mouse age terminology right. The mouse looked like a young adult, full fur and was roughly the size of his widest part (from what I could see through the tank). He had to work to get it down! lol.

The tank is a glass tank with warped black vinyl strapping around the top and bottom. It had a strip across the centre but it looked like it had been melted by a light and was severely warped so I cut it out. Was thinking about taking the whole thing off and replacing it with 1/4"x2" wood trim and attaching the plywood top to that with latches for easy removal?

Hard to explain but it looks good in my head!