Ok, I have no idea what is going on, but something in my pool is eating CYA for breakfast, lunch, and dinner.
Here's the full story for my pool. I purchased and moved into the house Dec 1, 2014 (6 months ago). This is the first pool I have owned, so I had no clue what I was doing, what equipment was what, or how any of it worked. I asked our realtor for a reputable pool service person to give us a once over on the pool and teach me how to maintain it. The guy we had come out was very thorough and gave me good information (lined up pretty well with what is taught here). It was a good starting point. He ran a series of tests and let us know the pool had FC=0 and that the CYA=>100 by a lot. On his recommendation I drained about half of the pool and replaced with fresh water, went to his recommended pool store to get some chlorine and regular testing to get the pool ready to swim in. After the first drain and fill session, the CYA came down to about 70, so I was told that was fine.
The pool is somewhere between 10K-12K gallons. Since that time, I have added about 10lbs of CYA to the pool over two main treatments. My pool is again nearly out of CYA (I've got reagents on order, so I used a test strip this morning that has agreed with my T-100 test and it showed it in the lower end of the 30-50 range).
Any ideas what would eat CYA that quickly? The stuff I have been using is from Walmart (Clorox Pool and Spa 100% CYA).