I don’t have a JBJ 65, but I did remove a glass partition from my own DIY sump. I used a razor blade to cut the silicone holding the partition in place along the sides and bottom (being careful to try to avoid damaging the silicone in the corners). After going over it a few times I could feel that it was starting to loosen, so I kept rocking it back and forth as I kept working to cut the silicone and I was finally able to get it free. Then I used the razor blade on it’s edge to remove as much silicone residue from the sides and bottom of the sump as possible.
It actually was a bit more difficult than I was expecting, but it wasn’t too bad. I did drain the sump to do this (and took the opportunity to clean out the sump at the same time) but it probably could be done with the sump full with a bit more effort.