Although I'm not familiar with the Brita product, other than having seen them advertised, I'd guess they have a prefilter and maybe a carbon filter. As this was posted in the FW
forum, I'm assuming you are keeping FW
fish? If so, why are you wanting to use purified water? Nothing wrong with that of course, but it's usually not necessary for freshwater fish. What kind of fish are you keeping? What is the pH and hardness of your tap water? You can certainly use distilled (water that has been boiled and recondensed) water as long as you put an additive in it to restore the mineral content. Something like "RO
Right" or a similar product. The reason for this is, although distilled water is pure and neutral, it has no mineral content and therefore no buffering capacity. The pH of this water will be very unstable and it will likely go acidic very quickly in it's pure form. If your tap water is very hard and alkaline, you can mix distilled with it to achieve the desired hardness/pH.