I'm a little disappointed with the remarks ref RO/DI water for freshwater tanks. I have a well and my water is totally unsuitable to freshwater or saltwater fish. So RO is the only way I can maintain a tank. DI is just extra special!
Some clarification for freshwater aquarium keepers with my problems. YOU want to get an RO unit, BUT, you have to use something like R/O Right to put back needed minerals, etc to make the water OK for your fish. The products that you can use will ensure you have appropriate KH, GH, and PH.
Many ppl say to mix your RO or distilled water with untreated water. NO. It is wrong headed to use an RO unit and then mix it with your well water, you don't have good water quality control if you do that. I am so happy that we have so many better ways to control our water now then we did in the 80's! Take advantage of them for crying out loud!
~D