Ok, a relative built a 40K pool in Mexico (note: hot, sunny). There is no filtration system; it is drained and refilled once every two months or so. He vacuums it occasionally (once every several weeks) using a siphon action that draws some water out as he vacuums. The only thing he tests is pH and chlorine levels.
The pH stays right at 7.6 without any maintenance.
Here's the kicker: he says he adds around 2 cups of granular chlorine once every 4 weeks or so and the FC remains at above 2ppm. And it's in the hot Mexican sun all day. I've been to his pool a couple of times; it is clear and NOT green.
Is there something I'm missing? Maybe something in the water?