The problem is with evaporation. If it was just a question of water changes, you might get away with conditioning your tap water, but top up is a different story.
Only pure water evaporates. All of the TDS from the top up water stays in your system.
Anyways, if you want a stable system, you have to control what goes in.
http://www.conceptaquatics.ca/index....er-100gpd.html
- Brad