Repaired our grey tank five years ago as it had a crack on the bottom, easy to get to. Used a kit from Plasti-mend:
http://plasti-mend.com/cart/index.ph...&product_id=96
These are photos of the repair. First photo shows the inverted V molded in the bottom of the tank and the crack is at the top of the V.
Second photo shows the repair made with the Plasti-mend.
Third photo shows the added braces with blocking between the bottom of the tanks and the braces. Braces were then removed, coroplast re-installed, and then the braces were re-installed. Tanks don't sag now and no more leaks.