Sounds like your filter is 'channeled' or has grooves in the sand that allow the water to bypass most of the sand. Also sounds like your sand if fouled.

Do this:

1. Being dosing with 1/2 gallon of PLAIN 8% bleach added manual EACH evening. You can do this while the pump is NOT running.

2. Remove all the sand from filter, and carefully inspect your filter parts, comparing them to the manual. Replace what's needed.

3. Refill with new clean filter sand.

4. Try again.

The bleach will 'keep' your pool, while you're working on the filter.