Why are propane tanks round?

The tanks are shaped in the best form to withstand pressure and prevent explosions. Round is strong.

Round Propane Tank

Propane tanks are round because a sphere is the strongest shape possible for pressurized gas.

You need to make sure your heater is set up for propane first. Most pool heaters are set up for natural gas only, running propane in it would be dangerous.

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Making corners would involve more seams and bending, more points of stress.

They usually are around in peoples yards..you don't want to hit your shin on the corner of a propane tank as you mow the grass or other....

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It depends on the BTU rating of the heater vs. the btu potential of the amount of propane. It may heat it but it probably wont last too long. I would see what the local pool supply guy says

Best way to hold the pressure

to prevent cracking, punctures damage etc

So they can roll.