Go to an auto parts supply store . NAPA auto supply or one semilar. Ask for Fiberglass Resin Jelly, # 765-1240, (pint size is samlest you can buy), and Fiberglass Redin # 765-1285 (NAPA part numbers) and some gause material, (piece about 12" square). The material they use for tape bedding sheet rock is the same thing and is alot cheeper. It has a glue substance on it, so it will stick to the fiber glass tank easly.
Apply the gause tape to the effective area, then after you have mixed the resin and jelly together (mix a small amount,because it sets up fast) and apply over the gause tape. Let set up according to the directions on the can, then apply a second coat. You will need to have the material overlap the area repairing at least one inch.
I have seen Bell Telephone Company do the same process when they wanted to keep moisture out of a line connection.
Good luck. Blue Moon