Phosphates don't 'kill chlorine', so no, that's not the problem. I'm guessing they told you 200 ppb phosphates, since 200 ppm would be extraordinarily high. 200 ppb is very, very low -- so they sold you something, just because they could. By the way, phosphate removers cloud up your pool rather badly.

Cleaning up algae uses HUGE amounts of chlorine, however. Most likely that's the problem. However without pool info (CHART!) and testing info, that's as far as we can go. Test strips (aka 'guess-strips') are very easy . . . and very inaccurate. But they're easy, so they are popular. And, of course, pool stores gain no benefit whatsoever from accurate testing -- if everyone had a good test kit, their chemical sales would drop enormously.