This is related to an earlier thread,
but I'll post it separately so its easier to find.
I had green water, three times this summer, first two resolved with 3-day blackouts, but it came back. The third time I cleared it up using advice received here:
1. I did 50%/30%/30% water changes and filter cleaning on consecutive days, to reduce excess nutrients that may have built up from fish, feeding, etc. I did these water changes without vacuuming, to not stir up more nutrients. Of course, the aquarium looked a lot better from simply diluting the green water.
2. I upped the ferts and CO2
to encourage plant growth, and did not prune. I also cut back feeding a lot.
The green water totally cleared up in about a week with this approach, without a blackout.
Setup: heavily planted and populated 29-gal
with DIY CO2