Using a heater with a small (barbecue) propane tank?

Our pool heater was connected to a large (120 gallon?) propane tank, but we only used it a few times a season (just for heating the attached spa, not for the big pool) and couldn't justify the tank rental fees.

I was wondering if I could adapt the small propane tanks (the kind you get at Home Depot et al) for occasional heater usage. Any pointers to technical issues I might need to research (pressure levels, regulator, connectors) would be appreciated.

A 4 gallon tank could heat a 11,000 gallon pool just 4 degrees F. How often would you like to change the tank?

I'm sure you could, but you might want to find a book on the subject or something. There may be different pressures and things that need to be adapted. I did the opposite; had the gas company run a line from my rented 120gal tank to my propane grill rather than filling the 5gal tanks.