You can use a heat gun for the top and bottom trim to get it to let go of the old silicone......otherwise it's easy to break getting it off there.
Also I would not use silicone purchased for hardware stores..... and if you're turning the tank into a sump, then get your baffle pieces cut to the proper size and make them a part of your re do process so that those panels will be a structural part of the sump and not just a divider.
This will greatly improve your chances of not having a "glass tank used as sump failure issues".