Koi

Ben Helm says...

A crucial element in pond design is to ensure that the water within a pond circulates though the pond and into the filter at least once every two hours. This means that both the dissolved and particulate pollutants are carried to the filter on a regular basis, leaving the pond free of pollutants and debris. A dead spot is an area of a pond where, due to the flow characteristics within that pond, debris is allowed to settle a accumulate. Settled debris on the pond bottom is indicative of a dead spot and will require you to adjust the flow patterns so that all debris is either taken to the bottom drain or pump so that it can be removed by the filter. Dead spots are best prevented at the design and construction stage, removing any nooks and sharp bends from the pond's design, ensuring that sides and curves are sweeping and that the pond bottom slopes down to the bottom drain(s). The installation of a turbo bottom drain can help remove dead spots in other areas of your pond by sweeping debris into the bottom drain.

Ben used to farm Koi for a living and has published three books on ponds. See his website watergardenersbible.co.uk for more details.