If you pour in 3 gallons of 6% bleach when your FC is 7, your FC should go to 20ppm, which is the recommended shock level for CYA=80. Cal-Hypo won't get into your water nearly as fast.

If your pump MOTOR is seized up, you can replace the motor. If the pump is seized up, it may be repairable.

But your pump's capacity MUST always be lower than your filter's capacity or you will ruin your filter.

My Dad ran a 13,000 gal AG with a 3/4hp uprated pump and DE filter combo for something like 15 years. A bigger pump uses more electricity, too.

Carl