Jan's right on the money.

Every pool store sells pool sand and it's not that expensive--on sale it can be as low as $5-$6 for a 50# bag and as much as $12 if they are gouging.

But I don't think you have any alternative unless something's clogged in your multivalve.

DE raises your pressure and that's part of your problem. You also probably need to shock, whatever the Bac proper shock is.

I have no personal experience with Bac. I simply don't see the point versus a properly maintained chlorine pool or, if you can manage it, an SWG system.

Good luck though!