Like a filter, the DI cartridge must be monitored for performance and replaced when it becomes exhausted. The best way to accomplish this is through use of a resistivity monitor. Another way is through the use of color change resins which give a visual indication of when the resins are exhausted. It is difficult to determine the exact replacement point of color change cartridges which often results in dumping of silica and other contaminants back into the water. Selection starts with understanding the feed water you will be using and the water purity requirements of the wildlife you intend to care for.
Water is a very complex medium.
In fact, it is technically a solvent and readily dissolves many materials which we refer to as contaminants. The method of removal for these contaminants determines what equipment and components must be employed in the design of the water purification system. An aquarist who lives in an area which has relatively problem free water and whose wildlife water purity requirements are minimal may only need a stand alone RO system. More sensitive wildlife such as corals and invertebrates will require the added water purification performance of a DI cartridge. An aquarist in an area with problem water may have to resort to difficult, relatively expensive solutions, especially if the wildlife water purity requirements are high.