With only 12 gallons you do not need anything extravagent in lighting. For the Basice you get get away with only a 24 watt set up however if your going dimmables then you want at least 30 Watts. You also do not have to go crazey on the heatsink as simple 1" X 2" Channel alumnium is more than enough for you without even fans.
The important thing is if you go with dimmables you want the blues and whites on swperate channels so you can dimm one without effecting the other. This is usualy a matter of dimming the whites to establish the color you like.
I would start with looking at roughly 18 to 20 watts of blue LED's. A good combo would be 6 Royal Blues, 2 True Blues and 2 of Rapids UV's all running on a 700ma driver. This comes out to roughly 20 Watts total of Blue lighting.
Now the general range for white lighting depends on your color preference but using neutral whites it ranges from a total of 5 Watts to 8 watts of total white power. If you go with cool whites then it is more like 7 watts to 10 watts of white lighting. I prefer the neutral white color with the blues so your looking at a low wattage variable drive. Unfortunatly Rapid does not carry these so you need to go with a fixed 30 watt driver if you stay with rapid running 700 ma. With this you can experiment running 2,3 or 4 white LED's to get the color you like the best.