RO and RO/DI systems help to remove impurities from your water: such as chlorine, chloramines, pesticides, phosphates, heavy metals and many others. Some of these impurities can lead to the growth of nuisance algae, while others may cause much bigger problems. The DI (De-Ionizing) cartridge does an excellent job removing phosphates, silicates and nitrates. They are highly recommend for reef tanks and will also work great for fish-only tanks. Numerous installation accessories are available for these units. The most useful accessory is a drinking water kit so you can also use the RO (or RO/DI) unit for drinking water.