I'm not sure if I'm using the correct # for my volume. 18 deep x 36 wide x 5.5 avg depth ( 3 in shallow and slopes to 8 in deep end) x 7.5(converting from cu ft to gal) = 26730. Then multiply by .85 for sloped walls in deep end = 22,720. Is this correct? I realize this isn't exact but am I in line using 23,000 for my calculations? Thanks and have a great Memorial Day.