68.75 gallons

__IF__the tank is standing on end Volume = Pi (3.14) x r^2 (r = radius = 1/2 diameter) x heighth. Diameter = 36" divide that by 2, square that number, multiply x pi, and then by 9" to find volume in cubic inches. 1 gallon = 231 cubic inches so divide by 231 to convert to gallons.