Deionization (DI) is a very convenient and cost-effective water filtration process for producing highly purified water on demand for aquarium or hydroponic use. By definition, deionization, often called DI for short, is the removal of all ionized minerals and salts (both organic and inorganic) from water through the process of ion exchange. Because most non-particulate water impurities are dissolved salts, deionization produces high purity water that is generally similar to distilled water, but compared to distillation deionization is much faster, less energy-intensive, and more cost-effective. In addition, it is an on-demand process that supplies large amounts of highly purified water as needed.

Deionization removes total dissolved solids (TDS) from water using ion exchange resins, controlling the electric charge of ions in the water to remove the TDS. Much like a positively charge magnet will attract a negatively charged magnet; DI resins attract non-water ions and replace them with water ions, producing a more purified water.

Demineralisation, mixbed systems, EDI (Electrodeionization) and CEDI (chemicallt enhanced electro-deionization) are the other methods to purify the water.