... ofplants but also oforganisms which areinvisible to the nakedeye and often forgot-Simple agitation systemsstir the water, enhancingthe diffusion of the oxygenrequired by fish. •12FRESH WATERten: ... further, you may alsocome across °Clark in older books on fish-keeping. The old-fashioned Clark system for hardness was somewhat laborious,being based on measurement of the foamcreated by a ... suspended, giving rise tomore pronounced turbidity.In aquariums, where the water is alwaysin motion, systems of varying degrees ofsophistication (see Filtration, page 237)allow fishkeepers keep their...