I think you are correct... the only time i've had water stains was from hard well water used in aquaria. This was for a time I lived out on a farm for 6 months.
Other than that, i've always had fish and/or amphibians using tap water treated with conditioners and have never had water staining problems, atleast nothing that couldn't be easily wiped clean.

I would think RO or DI water wouldn't leave any stain or residue.
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Thanks and take care - Joe Rollo
'Tis not the stongest of the species that will eventually survive, nor the most intelligent, but the one most responsive to change' Charles Darwin

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