Acuatic Plants for Dummies II

Hi all!!!

Well, i think i made it, some bumps along the way like rotting plants and some loosing all their leaves, but i think i got it now and i'm so so so happy, never been so happy with my tank before, so i got another one, and then another one LOL.

here is more less the graphic history of the tank

http://www.snapfish.com/share/p=855121081753394523/l=25835829

And now i have a question... i set up a 20 liter tank and hexagonal, and it has both acuatic and surface plants, it looks awsome, but my water is turning green, and i don't know what is causing it, the other 2 tanks never had that problem. it has a corner filter, a 20W heater and a "AZOO Galaxy Light". What is causing the water to turn green, and how can i make it clear again?

Best regards!
F.