Hi Rob,

You say that you've added stabilizer twice--what is your current CYA reading? (With the disappearing dot test, not strips). The two things that immediately come to mind are that 1)you don't have enough stabilizer in the pool, so you're losing all your chlorine to the sun, and 2) you still have some sort of chlorine demand going on in the pool that's using it up--we've seen a whole lot of that this year. What was your CYA level when you closed last year, and what was it at the beginning of this year?

You can rule out chlorine demand by testing your chlorine level at night, and again in the morning before the sun hits the pool, and comparing the two--if you've lost more than 1 ppm of chlorine, then you have something in the water that's using it up, and you need to shock the pool and maintain that shock level until you can pass that test. If there's 1 ppm or less chlorine loss, then it's possibly stabilizer.

Have you tried testing the water right after it comes from the cell (return stream) to see what the Cl level is as it comes back into the pool?