One of my tanks that has been up and running close to a year... one of the nicest tanks I have, started turning green about 2 months ago. Nothing has changed. The lights are on a timer. Plants not not changed. Stock has not changed. Regular water changes and filter cleaning, replacement. Then one day, green water.... after a few days it would get so green you couldn't see the fish. I would do multiple water changes in a day. It would clear up for a couple of days then turn green again. I cut the time on for the lights. Didn't help. I blacked out the tank with no lights. It cleared up in a few days but as soon as any light hit it, it turned green again.
I emptied the tank including gravel and cleaned everything. Re-resetup tank.... all was good for about 3 weeks then green water again.
I used API Algaefix... one dose. 24 hours later the water is perfectly crystal clear. It's still clear. No fish acted strange or had problems. It says not to use it on invertebrates. But I will have no problem using it again if needed. It's the first time in about 40 tanks and a few years of experience that I couldn't clear algae through management. I was reluctant to get it but it was a last resort and if I had known it worked that fast and that well, I would have bought it the first week I was having problems and saved me a ton of time and headaches.