It's not really that sunlight makes algae grow and flourescent doesn't, it's more that sunlight is a LOT brighter than the tank lights for the most part, not getting into things like different light spectrums and stuff. I haven't been able to find out what kind of lighting would be needed to match it, other than some idiot insisting that the sun is 10,000 times brighter than a 250 watt metal halide so you'd need 2,500,000 watts of flourescent lighting to match it...Ooops, tangent. Anyway, the sun is just a lot brighter than the tank lights so you end up with the algae outcompeting the plants sometimes. If you had good plants and constant sunlight though the same things to combat algae would work as with lower light.