Flex Seal repair on grey tank.

Unless it is a replica of the San Andreas fault, GOOP adhesive is as good a repair as any. The crack need to be dry. Place the nozzle of the tube hard against the crack, squeeze then follow the crack. Repairs have outlived my ability to keep track of them years later.

This stuff stretches like a rubber band and adheres tighter than a Time Share salesman. Scuffing up the edge area alongside the crack with 80 grit makes it stick even tighter. Want decorative results? Paint over top with RustOLeum black bed paint.