I've been using ultrasonic humidifiers for the nearly 2 years I've been growing orchids. I have hard well water and don't have RO. I collected some rainwater over the summer, but in the colder months I've been putting the well water thru a Brita filter. Supposedly it removes 85% of the calcium. It does seem to make a difference. The remaining calcium does cause the pin to "lime up" but I find that I can periodically scrape it off with my fingernail. I do have a fine white dust on the floor near the humidifiers which I'm sure is calcium. It also winds up on the leaves, but doesn't seem to bother them. I do have 2 muffin fans, connected in sequence so the breeze is gentle, to move the mist. So far I'm pleased.
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