It depends on a couple things.You really think the 36 watt light i chose is too much light? I've looked at every 18" cube thread on this site. There are two that get bumped frequently... One has a 150 watt MH and the other uses 48 watts of T5 HO. Both look great.
I have that 20" AquaticLife Fixture on a 16G bowfront which is either 16 or 18" tall. With no CO2 injection I was just blasting the heck out of the tank and getting some algae. I added a lot of floating plants and it dimmed the tank considerably. I haven't had any algae problems since adding floaters and I run a pretty long photo-period (probably ~14 hours most days depending upon my schedule).I planned on doing co2 and ferts. I'm always open to ideas though. It will be my first planted tank. Pc light options for this size cube tank? 18", 25 gallon.