DI (deionisation) resins are ion exchange resins which are used to produce deionised water or what we usually refer to as DI water. These resins are made up of small plastic beads, composed of organic polymer chains that have charged functional groups built into the resin bead itself. Each functional group has either a fixed positive or negative charge.
A small number of ions which are electrically charged atoms or molecules are found in water, even after the reverse osmosis process. The ions have either a negative or positive charge and made up of elements such as calcium, chlorides, magnesium, sulphates, iron, nitrates, manganese, carbonates, sodium and silica.
As the water in your RO systems leaves the RO membrane, the remaining dissolved solids are removed from the production water as they pass through your DI resin. As each ion passes through the DI resin, it is attracted to and attaches itself to either a positively or negatively charged bead, resulting in 100% pure, reef grade water with a 0 TDS reading.
DI resin is crucial and is used in all Reef Pure RO systems as it is the only way to ensure that you are getting 0 TDS production water for use in your aquarium. With a >98% rejection rate from your RO membrane, you will be left with at least 1% of the contaminants remaining in your water. Using this water without utilising a final stage of DI resin will mean that there is an unknown list of contaminants entering your aquarium. These could include heavy metals which are toxic to invertebrates of all kinds.