Ah, that makes sense.

I add bleach every single night, usually two quarts. I keep my chlorine on the higher end -- between 4 and 7 (CYA about 35, need to buy and add a bit more). Still, I get the green swirls (which I do promptly vacuum up but they only come back in a day or two). Since the water's always clear, I truly just surrender to the little swirls!