Xs Scuba Pony Pack Harness For Pony Tanks This Item Includes: XS Scuba PonyPac Harness XS Scuba PonyPac Harness Comfortable Harness Designed to Carry Pony Cylinder On-Your-Back Fits 13, 19, 30, 40 cu. ft. Cylinders Cylinder Diameters: 4" to 5.4" (10.16 cm to 13.7 cm)