Epoxy is best but you need an FDA rated grade for POTABLE water .....
--- kind of hard to find. If you dont use a rated for potable water
the leachables from the epoxy can be harmful.

In article , krj
wrote:

I need to repair a leaking seam on my fiberglass water tank. Somewhere I
think I read not to use epoxy in a water tank. Is this correct? Should
only fiberglass resin be used inside a fresh water tank?
krj