where is my CYA disappearing to?

[aylad]

There are bacteria that can break down your CYA and lower it--but that usually happens when the pool is allowed to turn into a green swamp. It is almost impossible to explain a drop in CYA from 30+ to zero overnight--except maybe by testing method. Who is doing your testing? And has the person doing the testing using the same method with the same lighting each time? Was a different vial of reagent being used? When the test showed zero, was the sample and reagent shaken well, then allowed to sit for a few minutes before being poured into the vial, or was it immediately poured in? All of these things can skew what is already a very subjective test, so I would rule out testing problems before looking for a problem with your pool.

Also, when you added your CYA, did you add it via skimmer, broadcast it into the pool, or put it in a sock in front of a return? If it went in through the skimmer, and you cleaned/backwashed the filter less than 4-5 days after adding it, then you may have washed it out. Four pounds of CYA in a pool your size should have raised your CYA way higher than 30.

Janet