You need to find why the tank leaked. My recomendation is to pull the ARF tank and replace it with a Sullivan or Dubro tank. If you tighten the bung down to much, it will split the tank at the opening. The Sullivan and Dubro tanks are not as likely to split or leak. You may need to do a bit of sugery in the fuselage to get the replaement tank in, but it will be time well spent.

Don