Math/New Water Help

[chem geek]

I don't think this is a situation of the soil bacteria getting into the pool and converting CYA to ammonia. It doesn't sound like the CYA dropped and in this case chlorine is getting measured -- it's just getting consumed faster than one would expect. When you have ammonia in the water, the chlorine gets consumed almost immediately -- I measured zero chlorine just 30 minutes later in my own pool when this happened to me.

To get an idea for how much chlorine might be needed until the water becomes stable, one can do a bucket test. Take a large bucket of pool water and add chlorine to it at a shock level. Measure the rate of decline in the FC and also note how much chlorine it takes until the FC holds when out of the sunlight.

Nevertheless, Carl is right that you should test all your water parameters, including CYA, since that's the only way to really know what to do next. If the CYA is at zero, then that would explain a huge drop in FC during the day, though it shouldn't drop much at night unless there is algae growth or something else consuming the chlorine.

Richard