Thank you both for the help. I battled algae earlier in the summer, and you're right about needing more chlorine. I need to pick up some more at the store. Has anyone noticed that the "gallon" size bottles actually only have 3 quarts in them?

Mostly I was wondering about why the algae grew with 3ppm Cl. Canuckpool must be right about leaving the cover on too long. I didn't realize it would be a problem because last year the cover was on 6.5 days out of 7 - the pool guy (which I don't need anymore thanks to this forum) - would leave it off whenever he came.