CYA 230, replaced 1/2 of water...now 217???

[CandyMoss]

Hi - the reason I detected the high CYA was that I was getting a rash from swimming...and so began the research and I was suspecting pseudomonas.

I had my water tested and was told that the chlorine isn't doing anything at all being that the CYA is too high. This might explain why after shocking a month ago it took 10 days for the chlorine level to drop down to 2ppm. I should mention that my water has no odors, no cloudiness, no green tints or algae - it's crystal clear!!

Background (before replacing water)
- 30000 gallon plaster pool
- Cl = 1.3ppm
- PH = 7.6
- Alkalinity = 160
- CH = 530
- CYA = 230!!!

I then drained almost half the pool, so I'd say I replaced 12000 gallons of water. Then I shocked the pool - and took my water back to the store for testing.

- Cl = 10ppm
- PH = 7.7
- Alkalinity = 100
- CYA = 217????

What the **** is going on here?
For one I don't understand how the CYA could only drop 13ppm after replacing 40% of the water. For two - can someone please tell me what to do from here? The guy at the pool store said "try replacing the sand in the filter". I don't know if he was just grasping at any solution....

Thank you,
Matz