These extra thick micro-fine polyester pads capture micro particles as the water passes through. Use Fluval fine filter fleece pads when cloudy water occurs or after aquarium maintenance for crystal clear aquarium water.Fluval Fine Filter Fleece for External Filter Series 04/05/06/07+ water polishing pads+ effectively traps micro particles and debris+ for crystal clear aquarium water+ for freshwater and marine water enviroments+ rinse frequently, replace after 1-2 monthsfor Fluval External Filter: 104, 105, 106, 204, 205, 206, 207 -Fluval No.: A242, 3 pieces packfor Fluval External Filter: 304, 305, 306, 404, 405, 406, 407 -Fluval No:. A237, 6 pieces pack