When you are testing throughout the day - is your chlorine dropping a lot? If so the algae can be re-growing while the chlorine level is lower than 20ppm.

I know that it can be frustrating - there was a person who had algae for about 3 weeks until she finally kept her chlorine levels consistently above 20ppm for a few days- then the pool finally cleared.

What's the saying around here - the answer is simple, but implementing the answer takes persistence and patients?