DI (Deionization) Filters - How do they work?

If you have a saltwater fish-only or reef aquarium, you have likely been told how important it is to have ZERO TDS, pure water. Replicating ocean water parameters is a delicate task, so it is important to start with as pure of a water source as possible to minimize the chances of things going wrong. Having zero TDS water also helps to combat algae problems and nutrient issues. The DI (Deionization) filter of your RO/DI system is what will get you that ultra-pure, ZERO TDS, water! 

It is impressive how a small canister filled with little plastic beads can do such incredible things.


These little plastic beads, DI Resin, are either positively charged or negatively charged. The positively-charged Anion Resin contains H+ ions; while the negatively-charged Cation Resin contains OH- ions. When water passes through the DI Resin, the contaminants are attracted to either the Cation or the Anion. Positive-charge contaminates such as Calcium (Ca+), Copper (Cu+) and Iron (Fe+) are attracted to the Cation Resin. The positively-charged contaminate adheres to the Cation Resin and -OH is released: creating an “Ion Exchange” or “Deionization”. Similarly, negatively-charged contaminants such as Phosphates (PO4-), Silicates (SiO2-) and Nitrates (NO3-) are attracted to the Anion Resin and H+ ions are exchanged. The resulting OH- and H+ ions released by the resins combine to form H2O, water! 

DI Resin

Here at AquaFX, we only use the highest-quality DI Resin that is made in the USA! All resins we use are virgin resin (not recharged/regenerated) and nuclear grade. Each DI Filter is also packed in-house, so you get the freshest, most-consistent and best-performing DI Filter possible.

We also offer several different types of DI Filters and Bulk DI Resins to fit your needs.  

The Mix-Bed DI Filter is the most popular. It contains a mixture of Cation and Anion Resins to purify a large variety of impurities.  

-The Silicate-Blaster DI Filter includes more of the DI resin that removes silicates and phosphates. These are great for water sources that have high silicate/phosphate levels. These are also a great option if you are having algae issues in your aquarium and want to ensure that you are not introducing any contaminants from your water that may be fueling the algae growth. 

- Anion DI Filter is great for multi-cartridge applications. As water quality varies, one resin can become exhausted quicker than another. With a mix-bed DI filter, you want to change the filter whenever either one of the two resins is exhausted. Having separate DI filters allows you to change out only the filter that becomes exhausted. The Anion Resin removes negatively charged ions such as Nitrates, Phosphates and Silicates.  

- Cation DI Filter - Similarly to the Anion Resin. The Cation Resin is perfect for multi-cartridge application.  

Most are available as replacement cartridges or can be purchased in refill/bulk packages. Refilling your DI cartridges with bulk DI resin can save you up to 50%! At AquaFX, we offer better filters, better service!