Need propane heater recommendation

I am replacing a broken heat pump. I am getting a propane heater because the heat pump took too long to heat the pool. For a 20,000 gallon 18x38 concrete inground pool, what brand and/or size heater would you recommend? Can you tell me how many degrees per hour heaters at various btus would raise the temperature? I think I need 300 to 400 thousand btus, but any advice would be appreciated. I want to be able to heat the pool quickly. Also, any advice on features I should have would be appreciated.
Thanks.

1 BTU raises 1 pound of water 1 deg F and 1 gallon weighs about 7.5 pounds. Therefore your 20K gal pool has about 150,000 pounds of water in it. Heaters are rated in BTU per Hour and you have efficiency and surface losses that complicate things but you can get a ballpark idea from this basic info.
Al

And...to add to the confusion....One Calorie is the amount of heat energy needed to raise one kilogram (or one liter) of water by one degree Celsius. The Metric system is far more unified. Volume, weight and linear measures are all designed to work together. So a kilo of water should be exactly one liter, and a liter is 1000 cubic centimeters--10x10x10cm.

But I won't try the conversions of pool volume....

BTW, the Rule of Thumb method of chlorine calculation works just as well if you substitute liters for gallons.