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. edox Redox, or reduction-oxidation potential, is a measure of how easy it is for organic reactions to take place. 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.