Try keeping it at 20 for another couple of days. There have been lots of reports this year of unusually high chlorine demand, as well as unusually high CC's. Don't know why this is suddenly happening, but the only thing you can do is keep hammering it with chlorine until you eventually break down the CCs. How much chlorine loss do you have overnight? (By that I mean testing after dusk, then again in the morning before the sun is on the pool). Anything swimming in it at night that you know of?

Janet