How to loosen a stuck bolt

That bolt under the tank is likely to have a rubber seal around the top, screw side, inside the tank. You must drain all water out of tank BEFORE you remove the bolt or your gonna have a lot of water on your floor. After you drain the tank completely use a screwdriver to hold the bolt from inside the tank to prevent turning while someone else is turning the bolt from under the tank. As long as the tank is drained anyway, if you can't keep the screw/bolt from turning inside the tank with a screwdriver, use a plyers to hold it from turning at the top. Take the old screw bolt to a hardware store and but a new one for only a few dollars. Once that screw-bolt has been loosened, even a little bit, it breaks the waterproof seal and cannot be re-used. Replace is the correct fix. After you screw the new one in place, place a "bubble level" at top of tank to make sure it's level. Be careful when tightening new screw-bolt. If you over tighten it, you could break or crack the tank. Looking at the screw from inside the tank, you should see that bolt head expand and seal just enough to prevent water from leaking from tank.

I am assuming that that screw bolt is plastic. The plastic is tough. A knife won't work cause the screw bolt is somewhat thick. You have to try to cut that plastic bolt at the point where it ataches to the bottom of the tank. I would try to use a saw and cut the bolt off where it touches the bottom of the tank. A hack saw would work best. If the room is too small for the saw, remove the hack saw blade from the saw and use it by its self. Any type of saw would likely work well, The tank is strong enough but I'd try to keep the blade from scraping against the tank too much. 

The best way to remove it, plastic or metal is cutting it off with a hack saw from below the tank. If you have a "vice grip" to clamp on to the screw part inside the tank. Vice grips gave a lot more grab than pliers. Do you have a vice grip?