I have 2.5WPG over a 12g tank. I am using DIY CO2, but still get a *little* green dot algae, and there seems to be a small amount of staghorn algae growing in my riccia, although it hasnt grown at all since i stopped it floating and attached it to a mat.
I do have a couple of apple snails, amano shrimps and bristlenose catfish that help keep the algae under control as well.