is reverse osmosis. De-ionization is a seperate stage that is optional, runs last after the RO
process is completed. De-ionization is the process of removing mineral ions. Deionization is a physical process which uses specially-manufactured ion exchange resins which bind to and filter out the mineral salts from water. It's highly recommended to really get pure, clean water for your SW
Like Mike stated, you can get cheaper units on Ebay that include the DI
stage. I'd look there first.