I think 6500 - 6700K CFLs (Daylight or [full spectum??]) is a must-have. I have two 10-watt CFLs over my 10g and, although the conditions of tanks vary, I'm on the edge of algae. A 20 long would be the same height as my 10, and the length I think is 24". So you're working with an extra four inches of length.
I think a 20 with two 13-watt bulbs suspended a little so you've got all the substrate covered would work - certainly 10s because of the same height.
MY TANK: Planted 10g; 2 x 10W CFL; Fluval U2 internal filter; MGOCPM/black sand cap
A Silent Cycled Tank
started April 18, 2012
Last edited by Django; 12-26-2012 at 02:35 AM.
Reason: changing data