what's causing the green water?

[Van down by the river]

To Mikey_918, Yes you are correct he shouldn't do water changes BEFORE treatment, but read the qoute again.

you may need to add temporary airstones or do additional water changes as the possible initial mass die off of the algae could cause low oxygen, killing your fish

If the algae content is very dense, the algae competes with the fish for oxygen. This can also cause PH swings (more of an issue in ponds). If he were to use an Algacide, or a good uv, there is a possibility that when all the algae die in a short time period it can cause problems. A bacterial bloom feeding on the dead algae again compete for oxygen. This is usually not a concern if it is a mild algae bloom.

Of course if you run a UV all the time and there is only minor algae within the aquarium this situation will never arise.

I would strongly recommend against the use of any algacides, cure the cause, not the effect.

Good luck ganowicki, overfeeding I find is almost always at the root or part of the most water quality problems.