You can use asetone to remove the adhesive. It can be excellent for removing paint, glue and almost anything else. While in a liquid form it is very toxic and the fumes as well. However it will evaporate 100 percent without leaving harmful residue. what you should do is use it to break down the old adhesive on your tank. then wipe off most of the acetone once you have done that wipe it off again using rubbing alcohol. You want to do this because once the acetone evaporates it will leave what ever it broke down behind which will be hard to clean off. rubbing alcohol will elimante this problem.

Also have you considered using wood for the rim? I use it on all the tanks i build. i just like the old fashoned unique look it gives the viv, and it is super easy to work with.