What if your cyanuric levels in your pool are above 300?

You typically want your cyanuric acid (CYA) level to be 30 - 50 ppm.

The easiest way to lower it is to drain and refill the pool. To get to 30 from 300 you would have to drain 90% of the water.

The only other way to lower it is with a reverse osmosis (RO) machine, which most people do not have access to.

The long term solution is to stop using buffered sources of chlorine, such as tri-chlor. These add CYA every time you chlorinate, but the CYA is never "used up" so it keep accumulating. Ultimately your chlorine is over-buffered or "locked" and is no longer available to perform disinfecting.