Abstract: Method and apparatus for reducing or eliminating the contamination by algae or phytoplankton in a fish farm (100). A series of tarpaulins (112) are suspended from buoyant members (103) which support the netting (102) of the farm (100). The tarpaulins (112) are connected at their lowermost ends (115) to a movable series of horizontal booms (122) that may take on air for positive buoyancy or which may displace the air with water for negative buoyancy. The booms (122) reciprocate on generally vertically oriented spars (113) which are maintained in position using anchors (104) and stays (111). As the booms (122) are raised or lowered, the tarpaulins (112) are similarly raised or lowered to allow or prevent algae from entering the farm (100). A water pipe (200) is maintained in position within each cage (101). Air is injected into the water pipe (200) from a compressor (214). The injected air initiates water flow upwardly within the water pipe (200).