Richard - ChemGeek - uses PR10000 and reports that PR10000 *does* remove PO4 as predicted.

But . . . what I'm seeing myself (I'm using it) suggests that it only does so AFTER you have lowered PO4 levels fully. What I suspect is that there are more phosphates in the pool system -- pipes, pool surface, filter media -- than there are in the water itself. This would not be surprising, since phosphates are often.added to water in the first place because of their abilities to attach to surfaces. If I'm right, then the attached phosphates will only be released back into the pool water when the PO4 levels begin to get low (< 500 ppb?).

My guess is that attached phosphates will be low in all plastic pools with cartridge filters (plastic liner, plastic pipes, plastic equipment), but high in concrete or stone pools with sand filters.

I would strongly encourage you, if you've reached 200 ppb, to go ahead and get that next quart. 200 ppb is NOT low enough to be very helpful, but it's very, very close to 100 ppb, which IS low enough. If you stop now, you are sort of like someone who started a 10k run, but stopped at 9.8k.