What I'm assuming Jason meant by "regular" is conditioned tap water. The reason you don't want to add this is the tap water might have thing dissolved in the water, such as excess nitrates, phosphates, etc. This, of course, would lead to those things getting into your tank. What you want is some RO/DI water. If you don't have an RO filter you can get them for some pretty decent prices (they range from 100). You can always just grab a bottle of Distilled water from walmart or other local store if you wish. Doing this however will cost you more money over time then just getting the RO/Di filter
