It takes a long, long time for CYA to break down under normal chlorination levels--I believe it's something like 2-3 ppm per month. CYA can and does break down under algae/swampy conditions--it breaks down into 2 or 3 different things, but one of the most prominent here is ammonia--which creates a HUGE chlorine demand to overcome.

If this OP will stop using stabilized chlorine products, the CYA will eventually come down on its own through splashout and filter backwashes--but it will take awhile.

Janet