Carl,
A fresh water tank leak is rare. It is usually the fitting or line from the tank that is leaking. However if the tank itself is leaking, the best repair method is to plastic weld a patch over the hole. This is also the best way to repair the fitting that is welded to the tank if it cracks, you weld on a patch and tap threads into the thick patch and install a pipe thread fitting into the threaded patch.
Gerald