I have a tank that in the summer months receives direct sunlight to about a quarter of it. Where the sun hits it grows algae like crazy. It looks like someone taped off a section of the sand and painted it green. The fish that I have in there love it and clear out about 75% of it. The next day it grows back. However this is not a planted tank because the fish also love to eat plants. I am tempted to move some fish around so that I can plant to see what happens.