An automatically skimmable tank for removing oil from water. A water inlet with a 90 degree elbow that is turned sideways and points slight upward causes flow to first impinge on the wall of the tank then flow upward helically around the tank's internal diameter (ID) to maximize retention time so that oil separates from the water. At the tank's upper fluid level, the water flows to the center of the tank, then spirals downward to the bottom and under the incoming fluid flow before exiting via the water outlet. Oil is removed from the tank via an upwardly extending oil outlet riser provided with two horizontal anti-vortex plates. A level transmitter senses the oil-gas and the oil-water interfaces and a programmed PLC activates an oil valve and a low shear oil transfer pump to remove oil from the tank when the oil layer surrounds the anti-vortex plates.