Some Filters actually incorporate carbon into their filters. I have a Marineland Millennium2000 hang on back filter. The filter cartridge has black diamond carbon in it and it works great because when the water passes through the filter it filtered biologically, mechanically, and chemically. But that Black Diamond stuff works pretty good at making crystal clear water. When I dont change the filter the water gets a little cloudy and when I replace it its clear in like 30 minutes.