I removed the rim from a 10G that I made into a sump. It's stuck on there really well and took a lot of work to get off. I heated the rim with a heat gun, then used an automotive windshield knife to gently separate the rim from the glass around both sides. You can't get to the top of the glass to cut that portion of the seal, so you'll have to use force to separate it. Heat it up until it's uncomfortable to touch, and slowly peel the rim off. It takes a good bit of force, even on a 10... anything too much bigger might pose a problem. Worked like a charm, and I did not damage the rim in case it needs to be re-installed for some reason.