Not knowing your tank or what its made of or if its accessible.. If it was mine, This might be my first approach..
1) Stop drill the crack on both ends. Drill a 1/8 diameter hole at the end of the crack, this will prevent the crack from spreading.
2) Clean the area around the crack, no grease, dirt, oils, etc..
3) Sand the area about 3 inches all around the crack with #36 sand paper. The sanded area needs to be scuffed and ruff. This will provide a good mechanical bond to the epoxy in the next step.
4) Use a two part epoxy, cut 2 pieces of Bi-Directional cloth about 3 inches wide and about 3 inches longer than the crack.
5) Using the two part epoxy, wet the area around the crack, then lay down two layers of the BID cloth. make sure the cloth is wetted.
6) Lay a layer of peel ply over the cloth, wet it out also. Let this cure and you should be good.
I used
West Systems Epoxy for these types of repairs. It and the cloth can be purchased at
West Marine stores.
I built an airplane out of epozy, so have a LOT of experience with it.
http://iflyez.com/
..