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. An optional coalescing media may be positioned in or proximate the communication port to absorb or coalesce contaminants as they pass therethrough.