I had a lift under the tank and removed the strap bolts. Began to lower the lift and the tank did not budge. I ended up pushing it up with the lift and then lowering it, repeated several times. The tank eventually freed up, no idea what was holding it.
They are held in VERY tightly to the body for safety and well other obvious reasons. Overtime they literally can get stuck to the floorpan, even with a full tank. Ask me how I know