You can buy the silicon sealant in a tube, usually at Wal-Mart, Home Depot, or other sources. You should find it some place in the home wares department, where ever they sell window or pipe sealant. I generally use DAP 100% silicone rubber sealant, and it comes in a 2 oz. tube. I would have to see some photos in order to help you any further. I believe the neck of the tank would be the area where the sealant is applied, but I'm not sure.

NorfolkSouthern