Even the purest tap water has hundreds of contaminants in very low concentration. However, when water evaporates from you tank, it leaves these contaminants behind. When you add top off water to make up for the evaporation, you are adding more contaminants, over and over again every time you top off. Eventually, those small amounts of contaminants aren't so small, and you can achieve toxic levels of some of them. That is why everyone here is recommending to you that you use RODI water - it is very pure, and you don't have to worry about contaminants accumulating in your tank. Using tap water for water changes is a different story. It isn't about accumulation of micro-contaminants, but the presence of things like phosphate, copper and other stuff. Phosphate encourages algae growth, copper can harm invertebrates, etc. People just want you to be able to avoid that. Water treatments remove the chlorine from tap water, but leave all the other stuff.