They have only one moving part, called the impeller. They are useful to create current within a tank or to drive filters such as under gravel filters , canisters and protein skimmers . This is a small mechanical filter attached to the input to another filter, usually biological . It is there to make sure that the biofilter does not get clogged with gunk, decreasing its effectiveness. This is an indicator of water quality, measured in millivolts with a special probe.
Higher values are better.
Redox controllers use an ozone reactor to raise the redox potential. A process for filtering tap water before it is used in an aquarium. This process generates water slowly and wastes a couple of gallons of water for each gallon of filtered water produced. However it is one of the easiest home methods. This variant of an under gravel filter runs in the opposite direction, pushing water up through the gravel.