I agree that buying
a new one is the best answer because (1) the old one probably has more weak spots than is apparent and (2) if you haul drinking water
you don't want to introduce patching materials that are not good for you. UV, heat, flexing, impact and abrasion all take their toll in time and you risk losing a precious water
supply. That said, we once painted a water
tank inside with melted paraffin. It worked but imparted some taste. Too, paraffin is highly flammable to be used away from the boat and with utmost caution.