I have a 20g tank that had the green water problem too. I did an entire water change, also cleaning everything in the tank but the green water still came back! I bought Kordon's AlgaeQuel solution and that cleared it up.
http://www.kordon.com/kordon/algaequel/index.htm
It's pretty overkill though, a capful can clean a 60g tank! If you have fish in the tank, the high temperature plus algaequel will distress them, making them easy turtle food. That happened to me
The stores around me only had the 18oz bottles, so look for it online for the 4oz bottles. I haven't had the algae issue again, so I have an $18 bottle of useless liquid now. Ah well.
There are also discs that you can toss in and supposedly clear up the water.