If you're getting your water from an unlimited supply like a pond, lake, or large cistern, you can go with almost any smaller pressure tank, but if your source is limited, we suggest basing the tank on at least three times your GPM requirements. For example, say you need 12 GPM. We would suggest a minimum 36-gallon pressure tank. Keep in mind, too, that you can never go too big on that pressure tank, as it can also act as a holding tank.