I am looking to purchase a 100K BTU propane heater for my 18' round pool and I have a few questions:

What size propane tank should I use? LBS?

What safety issues do I need to be concerned about?

Thanks to anyone that can help.

I have made some assumptions. First, the lowest temperature that you will be using is 0 degrees Fahrenheit. Second that minimum that you will let the tank empty before filling is 1/3. This determines the vaporization rate of the propane.

At 0 degrees you will need approx 90lbs of propane to generate 100K BTUs. Tank capacity will require you to have 3x90 or 270 pounds of propane in order to make the 1/3 full requirement. Tanks this large are generally specified in gallons, one gallon weighing 4.24 pounds. A tank can only be filled to 80% of its total volume so in this case you would need something around 80 gallons.

If you don't plan on using the pool below 40 degrees then a 100lb (28 gallon) tank would be sufficient.

Most jurisdictions require the fill and the pop off to be 10' from any occupied space or source of ignition. All of this is generally located near the filling port. If you live in a flood plain the tank needs to be anchored and strapped. You can check with your local propane company for the requirements. If your jurisdiction requires it they can generally take care of all the proper permitting and make sure everything is installed in accordance with codes as well as provide he proper tank sizing.