DI -- deionization -- filters are normally comprised of a color changing resin that turns from black to rust colored when exhausted. It is the final stage and for reef tanks the most important filtration stage. Many use ONLY DI for their water, however, the fairly pricey cartridges last much longer if you use RO first, and that is why most use RO/DI units. You don't want DI for drinking, but you do for your tank. Do some research and make sure you have DI, if not add a DI stage to your system, it's fairly straightforward and whoever sold you your unit should make that possible, or even swap the whole thing for you.