I used to make tanks back in the old days, acetone will clean but not soften. Xylene will remove along with a few layers of skin and a few aged brain-cells.
Adding over the existing will make you feel good, but ain't gonna do a thing to add strength as the old and new will not bond. They may stick, not the same.
if it's an older tank, it's worth a gamble to setup for a couple weeks somewhere to verify it's integrity. Garage/ carport/ patio. If you really want to break it down and reseal, use unwaxed dental floss to cut the old stuff from between the edge joints, apply xylene and clean. Apply silicone to joint edges and glue sides back together, then go straight into tooling the corners. A small cake decorating knife with a rounded tip works perfectly, set you back a whole $2 at the cake aisle.
I used jigs for frameless tanks, if yours has a frame and you can get it off without breaking it, you can reuse as a jig, or buy a replacement frame.
Once you are done, you need to let it sit for several weeks to fully cure with the temperatures we are getting into.
There were some high end tank builders that used glycerin products to 'glue' the tank joint together. I don't recall the exact name for the stuff, but you would not get it apart in one piece.
