With the AquaticLife Twist-In Compact 4-Stage RO/DI Unit DI Cartridge Add On Package you can add a second Carbon 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 Carbon Block Filter Cartridge.

Why Should You Use Two Carbon Cartridges?
Using two Carbon Cartridges will actually save you money in the long run because you can place a partially exhausted Carbon Cartridge before a new Carbon 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 Carbon Cartridge before it's completely exhausted. When using two Carbon 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 Carbon Cartridge in the second position.