I think I need sand for my sand filter it holds 300 lbs, first is 300 lbs overkill for a 27000 gallon pool and second how much does sand run a bag?

The reason I think I need sand is this, I went to purchase a knew pressure valve and it doesn't work, I took it off while the pool was running and the water whichj usually shoots about 10 or more feet in the air was barely even getting out of the filter, and this was after backwashing for 3 minutes and rinseing for about a minute. Any suggestions what my problem could be???