Re: to drain or not to drain
The simplest way to reduce your CYA is to perform a partial drain/refill. Assuming you drain 50%, you'll end up w/approx. 50 CYA. You should test your tap water for Alk and Cal. If you have hard water, you won't see much of a reduction in calcium.
My pool water and tap water Alk is 80-90 and my tap water Cal is 100 so I was able to significantly reduce my Cal from 650 to 350 by doing a 50% drain/refill. My CYA went from 50 to 20.
Re: to drain or not to drain
Most manufactures of SWGs recommend a CYA of 60-80 and CL 1-3 PPM. You might want to check your manual.
Also, there have been many post of CYA and SWGs which you may want to read. Some, including myself, believe that higher chlorine levels are not necessary with CYA between 60-80 because of the high chlorine levels in the cell. Currently, my CYA is about 70 and chlorine is 2.0 and I have yet to have any problems. Check out this thread:
http://www.poolforum.com/pf2/showthread.php?t=3185
We recently had a pool party of 16 teenagers. Although the pool was cloudy that night, the next day it was clear as can be.