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People need to understand that when water evaporates, it leaves behind anything that was dissolved in it. So as you top the tank off, the concentrations of various things increase.
Ideally, you would top aquariums off (to replenish for evaporation) only with distilled water or at least RO water. That way, you aren't increasing the concentrations of various minerals, etc.
And you still need to do real water changes to lower the concentrations of nitrates and other wastes that are being added to the tank by the fish and their food.