I have stopped using stabilsed chlorine tablets as the CYA level was up to about 62. It went down to 0 so I used some stabilised chlorine to up it a bit and it rose to 73. So back to Calc Hypochlorite only but despite this it has risen even more and is now 83. It has rained heavily in the last few days and the pool is up almost to the top of the skimmer so I am going to vacuum to waste tomorrow, which I hope will reduce the CYA a little. The pool is vinyl, inground, 63,000 litres. Ph 7.2, TA 100 at the reading just now. I really could do with some help about why the CYA is rising. I cover the pool with a solar cover when not in use to retain the heat from the air source heat pump. I find all this very worrying.
Without changing out water, it’s unlikely that the stabilizer reading would go down to 0 on it’s own, so I’d double check your test strips. The best way to reduce stabilizer or CYA levels is to waste some water out of the pool and replace with fresh water.