What size sand filter should I use for a 20,000-gallon pool?

Go with a 24-inch sand filter — it’s the standard size and handles the flow and turnover fine for that volume. You could go up to a 30-inch for slightly longer cycles between backwashes and more tolerance for heavy use, but it’s not necessary. The real mistake is going too small, like a 20-inch filter, which means constant backwashing and wasted time and electricity.