Every aquarist is familiar with external filters - they come in a variety of shapes and sizes and depending on the manufacturer they have different quality and price levels. However, the systems and filter media used were always similar and tried-and-tested components were simply improved. New techniques have been lacking in the last 30 years and could only be established partially or with restrictions. Ocean Free has succeeded in developing the Hydro-Pure technology after long and arduous research work. It removes dangerous compounds from the water and allows for a fish population simply a few days after the new installation. In combination with the 3-stage filter system, consisting of mechanical, chemical and organic filter material, it produces crystal clear aquarium water. In this process, two titanium plates facing each other release a micro current of only a few milliamperes. The emission, which is harmless for people and aquarium residents, simply enables low water electrolysis where tiny quantities of hydroxide ions (OH-) and hydrogen ions (H+) are released. A special electronic catalytic converter, called "Cata Pure", which is inserted as a cartridge between the titanium plates, speeds up the response and billions of hydroxide ions (OH-) and hydrogen ions (H+) are produced. Thanks to the high oxidation power of hydroxyl ions, ammonium oxidises to nitrite and nitrite then to nitrate. The oxidation and reduction potential of the aquarium increases.