With the AquaticLife Twist-In Compact 4-Stage RO/DI Unit DI Cartridge Add On Package you can add a second DI Cartridge to your AquaticLife Twist-In Compact 4-Stage RO/DI Unit.This package includes: (1) AquaticLife Twist-In Cartridge Head; (1) AquaticLife Twist-In Replacement Color Changing Deionization Filter.

Why Should You Use Two DI Cartridges?
Using two DI Cartridges will actually save you money in the long run because you can place a partially exhausted DI Cartridge before a new DI Cartridge and completely exhaust the first Cartridge before it needs to be replaced. Otherwise, to maintain pure water you would need to dispose of your DI Cartridge before it's completely exhausted. When using two DI Cartridges you will always want to place the older, more used up Cartridge first. When you dispose of the Cartridge in the first position, you should move the Cartridge in the second position to the first position and put a new DI Cartridge in the second position.