1. Using acid doesn't raise the CYA. Muriatic acid, diluted sulfuric acid, or the crystalline sodium acid salt of sulfuric acid (sodium bisulfate) all are used to lower pool pH levels; none contain CYA (cyanuric acid).

2. Why do you want to lower your alkalinity? Is it causing you a problem? If your calcium is low, and your pH is stable, alk=200 isn't necessarily a problem.

3. There's no telling what your CYA level *really* is -- test strip readings (even by an electronic gizmo) are horrendously inaccurate.

4. See the Best Guess page (link in my blue signature block) for an explanation of the relationship between chlorine and CYA, and why that's an issue with mustard algae.

5. Using borax is the easiest long term way to reduce your pool's tendency to get mustard algae. Once you add it, it's in your pool permanently. With a sand filter, you'll lose a little each time you backwash, but it won't take much to keep up. 12 boxes of borax and 4 gallons of 31% muriatic acid should get you there. That should be a bit more acid than you need, but with a Alk=200, your pH will tend to drift up anyhow.

6. Oh, yeah. You can't lower alkalinity without acid. Doing so would be sort of like flying without wings or making bread without grain.