DI cartridges are the best way to make DI water ( the pure water with low Total Dissolved Solids or without TDS) from tap water or RO filtered water through ion exchange process. They are filled with high purified nuclear grade ion exchange resin to deliver high purity water with minimum TOC background. Lanlang DI cartridges could work in 10' & 20' standard and big blue housings. In-line DI cartridge works without need of any additional housing.
And for aquarium keepers, the water quality is a critical element in keeping fishes or plants. While you use DI cartridges, it makes getting pure water simple and economic.