Abstract: A method of mitigating plankton from fish pens moves denser cleaning water from a selected depth into the pen and discharges it to flow horizontally across the pen, but with a downward component and at a sufficient velocity so that the cleaning water sweeps (carries) the plankton from the pen.

Abstract: Tank cleaning system is formed by an upper deck having a spout potion and a deck portion, injectors and a separation chamber having a conical bottom wall and a top formed by a waste baffle. The injectors connect the deck portion with the separation chamber so that the waste baffle and upper deck are axially separated to provide an outlet passage therebetween. The injector passages direct water through the upper deck and waste baffle into the lower chamber and impart a significant velocity component tangential to the chamber to the water flowing therethrough to impart a helical flow to the water in the chamber. A debris outlet passage is provided adjacent to the bottom of the lower portion of the chamber. The dirt laden stream enters the chamber through the injectors and dirt is separated by centrifugal action in the chamber and rejected through the debris outlet while the cleaned water passes through the outlet passage and is returned to the surrounding water.

Abstract: Tank cleaning system is formed by an upper deck having a spout potion and a deck portion, injectors and a separation chamber having a conical bottom wall and a top formed by a waste baffle. The injectors connect the deck portion with the separation chamber so that the waste baffle and upper deck are axially separated to provide an outlet passage therebetween. The injector passages direct water through the upper deck and waste baffle into the lower chamber and impart a significant velocity component tangential to the chamber to the water flowing therethrough to impart a helical flow to the water in the chamber. A debris outlet passage is provided adjacent to the bottom of the lower portion of the chamber. The dirt laden stream enters the chamber through the injectors and dirt is separated by centrifugal action in the chamber and rejected through the debris outlet while the cleaned water passes through the outlet passage and is returned to the surrounding water.

Abstract: An impeller pump for moving large volumes of liquid against heads of up to 1 meteris formed by a substantial circular housing having its axis substantially vertical in which an impeller is mounted for rotation on the central axis. The impeller has an maximum diameter substantially equal to the inside diameter of the housing and is formed with a hub and the plurality of blades symmetrically positioned around the hub. Each blade extends radially outward of the hub from a root to a tip and has a foil shaped cross section with a lift to drag ratio at least 75 to 1 at a Reynolds number of under 10.sup.6. Each of the blades is skewed rearward in the direction of blade rotation so that the apparent aspect ratio of blade is increased and to reduce operating noise. Each blade has an angle of attack of between 2.degree. and 6.degree. and has a rake in the direction of flow through the impeller of between 3.degree. and 7.degree.. The impeller lifts water through the housing and directs it into the fish farm.