Thanks for the explanation, I appreciate it.

It turns out my filter sand was too low, measuring 16 inches from the top of the tank. I added two bags of sand to bring it up to 12 inches. Now the water is beginning to clear as expected, and I should be good in another day or so. I guess over time it had slowly washed out with backwashing and such and finally got to the point where it wasn't working very well.

As for the strange results from my bucket test, I made a typo. The final result was 20ppm, not 200.