Yes, 12" of 11" flexwatt should be sufficient to heat a 20gal tank. Thats exactly what I'm using on a 20gal, and 2 29gal tanks right now. Be sure to use a rheostat (lamp dimmer) or thermostat along with it though. What I did was sandwich the flexwatt between a piece of Reflectix (foil/bubble wrap insulation) and rubber tool box liner(open weave rubber mat). The reflectix protects the surface underneath, and reflects the heat upward. The rubber mat separates the heat tape from the glass surface to help prevent burns in case the rheostat fails.

Steve