20w CF would easly fit and should be good..
there is not much space in a 6g hex tank..
I have a 4.5g hex next to my TV as a lamp with a single sword plant, grows so well it looks like a plastic plant, but no fish or filter, since the tank is narrow and high its perfect for the sword. The hood is also plastic but I screw a L bracket on the inside to fit the light, its not a 100% perfect fit but I didnt need to make a hole into the hood to fit the light that way