A propane tank (technically called a bottle) .
Is a 500 gallon propane tank still called a bottle? Now, if it was a stand up 100-150 gallon tank, I can see "bottle" being the correct term. Just the behemoth that is laying in my yard looks like it should be called a tank to me. Maybe bottle is the correct term for gaseous contents and tanks are for liquid? Inquiring minds want to know!