A leaky ballcock assembly can run your water bill up in a hurry, but it’s easy to replace. Get a new assembly at the hardware store, turn off the water at the supply valve on the wall, and drain the tank by flushing. Take the float arm and refill tube off and loosen the coupling nut where the water inlet pipe enters the tank. Unscrew the nut under the tank and remove the old ballcock assembly. Put the new ballcock assembly in place and reverse the process, replacing all the washers with new ones. Turn the water back on and check for leaks, make sure the float isn’t rubbing anywhere, and then watch your water bill go down.