Well the tank is 15 inches high but cedrick only has to fill it up 13 1/2 inches. All you have to do is figure our the volume of a box with a height of 13 1/2 inches and a width of 30 inches and a length of 10 inches. To figure out the volume you must multiply the length, width and height. so 10 * 30 * 13 1/2
