A 20 gallon high tank is roughly 24L x 12D x 16H. I don't think a 10" x 10" heatsink would be good for that. Much better would be a 18-20" long heatsink which could be as narrow as 2-3 inches. A piece of extruded aluminum channel would work fine, since you intend to use it at low current.
Using 6 Cree XM-L evenly spaced along that heatsink, say 3 inches apart, running at 400 mAmps should give you about 40 micromols of PAR at the substrate, with the light sitting on top of the tank, with no optics. Raise it about 6 inches and you should get about 30 micromols of PAR, good low light. If you zig-zag the LEDs across the top of the tank so you effectively have two rows, with them spaced about 3.5 inches apart on diagonals - two rows 1.5 inches apart - you should get about 3/4 of that PAR, so you could sit the light right on top of the tank for low light. Then the plants wouldn't grow to the middle quite as much, and the PAR from front to back of the substrate would be more uniform.