Why is a flow of clean water needed?
Fish require clean water flow to remove the wastes that fish produce and to replenish oxygen in the water that the fish need in order to breathe. The fish wastes consist of solid fecal material that remains after the feed is digested, and soluble, nitrogenous compounds, mostly ammonia (the end product of protein metabolism) that is rapidly converted to nitrate nitrogen in most oxygenated waters. Nitrate is the natural and principal form of nitrogen occurring in the oceans. Such wastes are produced by all animals and, in the case of fish, are dispersed by water flow and then assimilated by other organisms as part of the natural, aquatic, nutrient cycle. A similar process occurs on land when animal wastes are ploughed into soil. Comparisons of human (municipal) wastes production to fish production are misleading and incorrect for the reasons cited in answer 24.
