A reverse osmosis water filter system requires the use of a RO membrane and one or more filters. Most residential reverse osmosis water filter systems have one filter, then one RO membrane, and then another one or two filters before the water enters the product water tank for storage. A reverse osmosis water filter can be filled with an array of different media, depending on the water quality and application. For instance, a residential system used for drinking water will generally have a carbon pre-filter, a RO membrane, a carbon-post filter, and a sediment post-filter. But if the reverse osmosis water filter system is intended to be used for an aquarium, a carbon pre-filter, a RO membrane, a carbon post-filter, and a DI post-filter would be recommended.