06-28-2011, 09:14 AM
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Ideally, if one was to correctly compensate for excessive evaporation, distilled water would be the best choice, especially if the water contains a fair amount of mineral content (as these would be concentrated).
However, most of the time, with moderate evaporation, the weekly water change is typically sufficient (if not ideal) to deal with 'freshening' the tank water. In the case of high evaporation, perhaps a greater water change is warranted.
If the water supply is chlorinated, of course one should never add un-aged or untreated water to the aquarium as it may upset the biology and/or prove lethal to the stock.