If your tank is ABS, the black plastic material that is the same as the black plumbing pipe used for vent and sewer piping, you can use Plasti-mend. I repaired a crack in the bottom of our grey tank with one of their kits, and it is solid, no leaks. I don't really think any tape product will hold for the long term.
http://plasti-mend.com/cart/index.ph...tegory&path=59