I would say that if you have a tank leaking and you know about it. Then it would be highly advisable to get it out of the ground immediately, especially if it is holding petroleum based product. I am not aware of any type of sealant that would be approved to perform this type of "leak stoppage". The Steel Tank Institute (STI) that governs most in ground tank storage would be your best bet.
But my general thoughts are that is this tank is located in the United States and you think, or know it is leaking, removing of the tank is the only option.