The local Aquarium is looking to add a new 8 ft. deep dolphin tank. They want to choose a style that holds less water to save on their water bill. They can either install a circular tank with a diameter of 30 feet, or a rectangular tank with the dimensions of 30 ft. by 20 ft. If the dolphin tank holds 7 gallons of water per cubic foot, which tank should they purchase, the circular tank or the rectangular tank?