Illin, welcome to the forum, you've picked the right place to find no BS answers to pool care (not to say that BS never shows up here, but it gets cleared out quite quickly). Unless you have your pool over a stream, you should be able to drain 2 1/2 - 3' at a time without fear. If you use the sheet method advocated here (get a really big plastic sheet - enough to cover the pool with a ~5' overlap all around- and put a pump in under it to remove the 'unwanted water' and turn on the garden hose on top of the plastic. The plastic keeps the fresh water from diluting the water you are pumping out, and refills the pool at the same time * Don't use this method if you have 'fill water problems' (metals or whacky chems in the tap water.
Good luck with the cya lowering, hope this helps. If you need more help, all you have to do is ask! -Waste