Awesome, fast, helpful answer. Thank You!

And a followup or two, if I may.

Do I buy the DE at the pool store, or the garden shop? Will it rinse out of my sand filter the next time I backwash and rinse?

I backwash and rinse weekly for general purposes. My pressure gauge on the filter is broken (it got COLD here last winter)...is it worth it to replace it annually?

Will the cheap OTO kit tell me how much chlorine to add to achieve 15ppm? And until it stabilizes and drops back to 3-4pm, I'm guessing the pool is closed?

I am behind the curve on the learning I need. I was probably typical: take a sample to the poolco, buy whatever they sell me and dump it in the water, rinse and repeat until the water looks good. No more.