How to either cheaply heat a pool, or heat a pool for one day?

Heating a 6000 gallon pool is a tough chore, heating a 15,000 or 20,000 gallon pool quickly is tougher.

The goal is, as Poconos says, to pump BTUs into your water. One BTU is the amount of energy to heat one pound of water 1 degree F.

10,000 gallons is over 85,000 pounds of water. That means you need 85-90,000 BTUs to raise your water 1 degree. You'll need 10-15 times that to get you water just to the mid-70's from the mid-60's

Heaters are rated in BTUs per hour......For one day? I have NO idea how much NG or Propane that would take.

Just my take.

Carl