Distilled water?

[MizTanks]

Good or bad for SW?

[waucedah_joe]

Other than the possibility of it being condensed in copper coils, I don't think there's anything wrong with it. It's just expensive. Best to get your own RO/DI filter.

[AZDesertRat]

If you are using water in small quantities distilled is a good choice to purchase. The fear of copper is an old one, commercial stills have not used copper in dozens of years. Most are now titanium or epoxy glass lined exotic metals so corrosion and copper are not issues.
Long term, as already mentioned, it can get expensive and having youir own RO/DI system is the way to go. The average cost of making RO/DI at home ends up less than a nickel a gallon including water and sewer rates and replacement filters, plus they can be used for RO drinking water too which eliminates billions of plastic bottles in the landfills every year. Did you know over 50% of all bottled drinking water in the US comes from municipal water plants and not springs or special sources today? Why pay for something you can do yourself?

[BeakerBob]

Distilled water can be used with no issues, especially when added to salt mixes. Distillation removes the minerals from the water as well as many of the volatile organic substances. Using it to make up your salt mix will replenish the lost minerals needed for the tank water.

Distilled water is a much more intensive process in terms of energy use as the water must be boiled and condensed. Reverse osmosis and Deionization (RODI) is a much cheaper process and does not involve a lot of equipment.

[AZDesertRat]

RO/DI will also be a little more pure since some volatiles can make it through the distillation process but not much if it is done correctly.