The Aquatic Life RO Buddie 50 GPD and 100 GPD 3-stage reverse osmosis (RO) and 4-stage (RO/DI) systems feature unbelievable affordability and functionality, all in a very compact design. Designed to provide a reliable source of economical filtered water for aquarium use, all systems use Thin-Film Composite (TFC) Membrane RO cartridges in conjunction with Carbon and Sediment cartridges to effectively and efficiently remove harmful substances such as heavy metal ions, dissolved solids (TDS), Chlorine and particulate matter from municipal tap water. Features: Small footprint of the unit makes it easy to place under cabinets or in limited spaces - Sediment and Carbon cartridges are up-front and easy to replace - Membrane Housing accepts standard 11-3/4 inch TFC Membrane Filter Cartridges - Faucet Adapter makes it easy to connect to standard Garden Hose Threads (GHT) - Quick Connections make it easy to remove and replace cartridges - Mounting Brackets hold the unit securely while mounted in a convenient location - Included Membrane Housing Wrench makes removing cap easy. Note: User must supply the 1/4" tubing to connect to the incoming water, filtered water and waste water connections on the RO Buddie. The 4-stage systems include our improved DI Color Changing Mixed Bed Resin Cartridge with MBD-30 resin, item #330242. Both RO Buddie 3-Stage systems can be easily upgraded to a 4-Stage RO/DI system by adding our De-ionization (DI) Color Changing Mixed Bed Resin Cartridge, Item # 330242, sold separately. The Cation and Anion resins (H+ and OH-) in the DI cartridge media remove most of the remaining total dissolved solids (TDS) from membrane-filtered water, and produce purified water with an average conductivity value of ~0.1µS/cm. The DI Cartridge is refillable and has a clear housing so you know at a glance when the media needs to be replaced. Our new MBD-30 resin changes color from blue to yellow, then tan and finally white when the media is exhausted. Each DI Cartridge includes two clips that allow the cartridge to attach to the front of the membrane housing. 50 GPD RO Buddie (540016), 100 GPD RO Buddie (540017), 50 GPD RODI Buddie (8000024), 100 GPD RODI Buddie (8000025).