It's interesting how this thread has morphed from testing for phosphates to testing the whole need to do so in the first place. I guess the answer (for me anyway) is it doesn't seem likely the effort is worth it. I'll just keep my FC at the upper end of the best guess chart, raise the CYA to about 60 and see how it goes.

It has been worth the effort to discuss the idea here. Thanks for all the expert info.

Happy 4th all

Splash