I've been curious about this myself. One of my tanks loses from 3/4" to 1" in a week.
This is a cycled, 5 gallon tank and I thought I read that distilled water could be added, but that over time dechlorinated tap water used as a top off would start to raise the hardness?
On smaller, unfiltered tanks where 100% water changes are done it wouldn't have the same effect.
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