Had the same problem as the bolts were impossible to take out. Lucky for me the bolt was intact as no one had cut them flush to the nut. I put my vice grips on the very tip of the bolt and then used a small wrench to unscrew the nut till it hit the vice grip. I then used needle nose vice grips to grab the bolt above the nut and finished unscrewing it. Be careful if you decide to cut the bolt as the risk of the tank breaking is high if it lets go when the bolt is cut.