A gas flotation tank is provided that includes a series of adjacent chambers which impart a rotational current therein. Each chamber is separated from a skim oil trough by a skimming weir. Each chamber comprises an alternating fluid communication device between adjacent chambers allowing fluid communication between adjacent chambers in the form of a communication port in the dividing wall between adjacent chambers and a chamber outlet in conjunction with a perforated plate and the outlet is positioned in fluid communication with the final chamber.