Vise grips, channel locks, anything that will get on the big nut will do. If you don't have the room or your back isn't up to it or it's stuck it may be easier to take the top tank off. A screwdriver and a socket on two bolts takes it off. You can get a kit of new bolts, washers and tank-to-bowl gasket for just a few dollars. New parts will help prevent future leaks.