Hi, thought I'd put my 2 cents in here.

I had MAJOR green algae headaches!

I tried reducing/increasing fish load/plant load. Didn't work.

Tried diatom filter. Worked initially, but green water returned in a
few days. Then wouldn't work at all.

Tried diatom filter with something to particulate smaller algae.
Worked - but green water would eventually return.

Tried Daphnia. Didn't work (but mollies enjoyed tearing holes in
nylon bag containing Daphnia and eating them up).

Tried UV sterilizer (at the suggestion of Jim at The Bug Farm whom I
had called to buy more Daphnia - THANK YOU JIM!). Worked like a
charm. My tank has never looked better. I bought a small 8 watt
sterilizer (for a 38 gallon tank). Perfect.

Good luck! Cindy