1) Get an electric glue gun and fill the hole. Put the screw back in place and tape in place until glue sets up.
2) Buy a "fatter" sheet rock screw (the key is finding a screw that has very coarse threads) and shorten the length with a grinder so it won't go through the tank. If you go to a larger hardware store you should be able to find a lot of varieties of screws to choose from.