I am a CPO by the www.nspf.com

I control a private swimming pool in NW FL pandhandle. I am a tennant in my apartment complex and have a 45,000 oval VAK-PAK DE pool with scum ledge. I was informed in the CPO class that the only way to remove CYA from the pool is to exchange the water for fresh water. Of course my other comment about the top 3 inches is not to be taken in the form that if you test 6 inches down you get a ZERO readout. The 3 inches of the top of the surface is where the CYA does its job. I cannot make a scan of the page out of the CPO manual. That is against the Copyright rules. I can only tell you what is says. And I have done it before.

My scum ledge is 6 inches high. Than accounts for about 1000 Gal. so I shut off the main pump and crank up the maint pump and evac from the ledge drains instead of the main drain and it lowers my CYA level from 90-100 to about 70-80. I fill with fresh water, wait 2 days and do it again. It works.