I'm a newbie with a new 20L (30x12) and am not sure what Coralife Aqualight to purchase for a lightly planted freshwater tank. I was thinking of the 30" single freshwater Aqualight that comes with 1-24" 65w PC bulb. My question is, is the 24" bulb centered in the fixture, or is it 24" from one end, leaving 6" of empty fixture? My other option is to buy the 24" fixture and use legs to center the fixture over the tank. Suggestions?