Just a couple of quick notes;

1. Your filter is too small for a 21k gallon pool. It will keep your pool fairly clear when everything's OK, but clean up will be VERY slow. The Waterway spherical tank design just makes things worse.

2. Your pump is WAY too big for that filter, and will force dead algae and such THROUGH the sand. It will also cause you blow sand out of the filter on every backwash. There's a good chance you are missing a bunch of sand.

3. You need to check make / model numbers on the pump and filter so I can confirm what I told you, above. => http://pool9.net/pf-equip-form/

4. If you purchase this filter, and install it BESIDE your existing filter, you'll be able to clean your pool much more rapidly:
Hayward Star-Clear Plus Pool Cartridge Filter
5. If you have a 2-speed pump, it will be easier to correct the problems.