I have been fighting with the green water for about a month by today.
I have a 30 gal fresh water planted tank with 2x55 PCF bulbs. I tried so
many methods, I can't tell you.... Almost lost my hope.
Here is what I have tried:
- daily change of water - up to 40% a day - the water clears for a little
while, then green water comes back, so thick, I can't see anything an inch
away from the glass
- reduce light - didn't work
- reduce feeding the fish - didn't work
- add more CO2 - didn't work
- darken the tank completely for 4 days - green water goes down a little
bit, then takes over again.
- bought a micron filter (Aquaclear Quick Filter) with Aquaclear 302
powerhead - no effect. The green water seems to consist of extremely fine
particles that even go through a micron filter easily(some people say
however that diatom earth will help, but I didn't want to spend a fortune on
a Vortex).

And yesterday I found this incredible solution! I went to Home Depot and
bought Beckett Pond Clarifier in the pool treatment section. They claim it's
Phosfates free and harmless for fish. Came home, poured it into the tank
according to dosage, and I couldn't believe, it cleared completely in the
matter of 2 hours in front of my eyes.

If you want to try it, don't forget to put inside the tank QuickFilter
cartridge - I guess it clears out suspended green water particles.

The solution was dirt cheap - only $10 for a bottle enough to treat a house
pool - so definitely more than enough for the fish tank.

Anton.