Six Stage RO/DI Aquarium System 75 GPD Membrane 4 Stage R.O. Dual DI REFILLABLE Color Changing Media Cartridges. ALL CLEAR HOUSINGS. Manual Flush Valve. Reverse Osmosis systems, combined with A DI after filter, will give you the lowest TDS reading for your aquarium water supply. DI Resin is necessary to reduce Total Dissolved Solids (TDS) to zero and is the last stage of an RO/DI unit. It is unnecessary to use the expensive proprietary DI cartridges that other RO manufacturers sell. Simply buy a refillable DI cartridge and fill with DI resin. You'll save money and get the same results. A constant supply of high purity water can be obtained by using two cartridges in series and allowing the first cartridge to completely depleted. During this operation, the second cartridge ensures that the water leaving the system is of the highest purity. New cartridges are placed into the polishing position and the remaining cartridge is put into the “roughing” position. This ensures complete utilization of all the exchange capacity, which lowers operating costs while increasing average quality.The clear cartridges are refillable, so you can save with buying media in bulk, and using the clear cartridges again, and again. Exported By ExportYourStore