Under mechanical filters, one understands cheap filter bags and filter sacks which are hanged directly on the outlet of main tank in the technology tank. The filter sacks should be cleaned as soon as they are clogged. Professional variants are the so called fleece filter which removes suspended solids from the aquarium. So the water is filtered over a fleece and continued over a roiller as soonas it is clogged. Fleece filter can also hold back parasites and ensure crystal-clear water.