i was thinking about using DI water for my tank too because i work in a lab. but here's what i found on the web:
"RO and DI are two methods of purifying water. This removes the impurities and any particulate that could be in the water, or neutralizes the pH by eliminating the various ions that could be floating in the water. While this gives you very pure water, this water is potentially dangerous for your aquarium. With all the impurities removed from the water, the water's ability to maintain its chemical properties are compromised. This presents a risk that the water could make sudden and dramatic changes in
pH, alkalinity, and hardness when exposed to any kind of chemical - even the aquarium equipment. It is important to know how to balance this water appropriately if you get it to make sure it will stay stable. In addition, RO or DI water will have most, if not all, of the trace elements necessary for your fish and plants to remain healthy removed, further prosing a risk to your fish. It is important to get the correct supplements for your fish or plants, and know how to add them to the water and how to test that you have the correct concentrations. Though RO or DI water can be very useful to advanced aquariists with the experience and knowledge of the water conditions they are trying to duplicate and how to duplicate them and test that the water is still safe, I would not recommend RO or DI water to a beginning fish keeper."