Thanks for all your replies! Gin, I do water primarily with tap water. The water in my area is slightly hard but not so much that I've had problems with any plants other than those on treefern mounts. I have a filter (not RO) that reduces minerals a bit but this time of year it comes out so icy cold that I've been using warm tap water to not shock the plants. I think I will start doing a distilled water soak every week as you and several others have suggested.
Steve, my best guess for the Sprite suggestion is that the mild acidity (carbonic and citric acids) would help solubilize alkaline minerals like Ca and Mg wihtout damage to the plant. The guy who told me this designs a lot of hydroponic systems and mineral build-up is apparently a common problem in these. Next time I get back to that store I'll quiz him a little more.
John, that's a good suggestion, I might try using some low-sodium seltzer water to see if it works any better or worse than plain distilled. I don't
think the sugar content of Sprite would be directly harmful to an orchid; my Dad (a botanist and orchid grower) has told me a few times that a sugar water soak will induce some plants to initiate new growth by giving them an energy boost. Of course any that's not well rinsed off could induce a similar explosion of fungal and bacterial growth...
Erce, do you use bleach to clean mounts that have plants on them? How well do the plants tolerate this? I'm a little scared of bleach!
Thanks again everyone!
--Nat