Looks like the rear baffle seal is leaking. Personally I would pull the rear baffle and reinstall to prevent future leaks, though this is much harder (need heat and make pulling tools). When you pull the tank, you will see how wide spead the problem is. If limited to that area, the repair can be done through a hole in the rear baffle. Remove old sealant, clean and scuff before applying new sealant. I WOULD NOT try sealing from the outside. Someone once brought me a tank with a half pint of proseal all over the outside of the tank and it was still leaking.