Best advice is a long screwdriver and spraying the bottom of bolt with penetrating oil and let stand for a couple hours, then try to remove.
Worst case scenario is hacksawing the bolt out, but that takes a talented technique to do that.
Use all brass tank to bowl bolts, nuts and washers on the tank, stud the tank, and use plumber’s grease, a small amount when installing the tank back on.
I always lip the underside of rubber washers with Megaloc teflon thread sealant to better seal the rubber to the bottom of tank inside.
“Your best interest is secured by making the right decisions the first time.”