Another method of getting rid of green water is the willow method. I personally think that this method should be used before the black out method and black out should be used in the last resort.
With this method, it takes 5-8 days to completely get rid of green water. It removes the cause of Green water and some other algae. You don't need to stop dosing your fertilizer and you don't need to switch your lights off. You donít need to do extra water changes, just do your normal water changes. Remember I spoken in the black out method that green water is usually caused by a combination effect of Excess Nutrients and Excess Light. While the Black out method is to get rid of the light source, this willow method is to get rid of the excessive nutrients.
My methods are as follow,
Step 1: Pluck a few branches from the willow trees.
Step 2: Pluck out the leaves from the bottom of the branches. So the tip of the branch will have some leaves while the bottom (the part where will be soaked inside the water) will be leafless.
Step 3: Put the branches into the tank with the part without leaves soaking inside the water.
Step 4: Wait patiently for few days and you will see roots starting to appear from the branches.
Step 5: The Green water should be completely gotten rid off after a week and this effect usually come overnight!
Step 6: At this point of time remove all the willow branches else your plants will suffer from the lack of nutrients.
Side note: I find it effective to put willow branches into a fish bowl with a lone betta in it. It helps in reducing the ammonia level in it.
If you do not know how a willow tree look like, check out http://en.wikipedia.org/wiki/Willow
hope this will help :D