So, did a full test on Friday to open the pool side for swimming - we use the spa all winter long. Water was clear, but I noticed that the CL level seemed low considering I had set the cell at 100% on a 5 hour a day run. I soon discovered why: for the 2nd year in a row, CYA was zero. Added 4lbs to get it up to 40. Had to do the same last spring. What would cause CYA only to disappear over the course of a year?
My salt level is actually creeping up
My borate level is stable
My calcium is also creeping up (around 750 now - I'm in AZ, so 750 is normal here)
11k pool/spa combo, SWG, pebble sheen finish, cartridge filter
Everything I've read here says the only way to lower CYA is thru loss of water. I have no leaks, minimal splash out, and don't backwash. In addition the salt and borates are mostly stable (salt is around 3800 and it started at 3200 almost 3 years ago).
What could chemically be causing loss of CYA?