Pea Soup anyone?
Perhaps nothing in the hobby is more frustrating than having a condition in your tank where you cannot see your lovely fish. It is very easy to obsess about this and go nuts pulling your hair out trying to rid your tank of this infestation of single-celled algae.
If you sit back and think about it, the only real problem that green water produces is an aesthetic one. The fish in your tank don't care one bit how green the water is! In fact, the subdued lighting due to the green water blocking some of the light is probably appreciated by most fish. So we need to put this situation in a proper perspective. Sure it doesn't look good, but the algae is there for a reason. Instead of seeing it as a plague in your tank, you should try to see it as a sign of an imbalance in your tank.
The only real problem that green water produces is an aesthetic one.
This is one of those times where I need to emphasize the importance of having a complete test kit. It's always better to know exactly what the state of your tank is, than to just speculate about the water chemistry and act upon those speculations. It's like flying a plane blindfolded... very dangerous!
Here are some basic tips on what to do in order to rid your tank of the green water. There isn't a sure-fire method guaranteed to work, as it may be a combination of things that finally make the green go bye-bye. Sometimes doing nothing at all will cause the problem to cure itself! The options for you to try are divided into into different sections: Universal Tips, Fish Only Tanks, and Planted Tanks. This is because your options may be different depending on which type of tank you have.
These options may be tried whether you have a Fish-Only tank or a Planted tank.
When you only have fish in a tank with plastic plants and some other decorations, the options you have to get rid of green water are more 'chemical' and less 'natural'. That's okay, because it can make things a little more simple.
In a planted tank you need to play by nature's rules. Algaecides and a 3-day blackout for your tank may hurt the plants more than the algae. Since algae control is nutrient control (did I say that already?) what we need to do is control the nutrients. This can be done in a few ways, either regulate the amount of nutrients (direct), or lower the chances that the algae will get to use them (indirect).