Where'd the CYA go?

[PoolDoc]

Two nights in a row I've added enough chlorine to bring the pool up to the shock region on the BGC, and both nights the pool tested much, much lower 2 hours later.

That's pretty definitive evidence that your pool had some CYA that got bio-degraded to ammonia or urea. But unless you want to take your water to a EPA certified lab, and have them do tests for COD (Chemical Oxygen Demand) and ammonia, we don't really have any idea how much ammonia is there, and consequently, we don't know how much chlorine it will take to clean up. We've seen it take 100+ gallons of household bleach on several occasions, when the CYA was very high the preceding fall.

But, knowing doesn't change anything: you're limited in how much chlorine you can prudently add at one time, both for the sake of your liner, and also to avoid the situation where you add a 50 ppm dose of FC when you only have 5 ppm of chlorine demand left!

By the way, keeping the pH on the high side of things, helps push the process toward less noxious intermediate products, as the chlorine breaks down the ammonia (or whatever is there in your pool).

[usmarinemike]

Working the pH up a little at a time. The owner used trichlor last season, and I assume every season because the floater was pretty beat up so the CYA was probably pretty high. I suppose 100 gallons isnt so bad. Great Value is about $1.50 this week. This will be one clean pool once the Cl holds.