Are you sure that the leak is comming from there, and not from above from say a leaky filler neck? Mine has leaked there for a long time, but I have never gotten around to fixing it.
If it is the tank leaking, they make compounds that you can put on the tank that will harden and glue the thank shut. You can check it out at the local NAPA for these products; or just drain, drop, clean, and JB-weld the leak.
-Rich