Hi, Reka. Just not much to write these days, I suppose.

If you're not getting root burn or loss of root tips, and the
growing tips are long, green, and otherwise healthy, what's the problem?

In terms of specific media recommendations- media that will absorb
more water will present more problems with hard water. For example, a
sponge that holds 10x its weight in water will have a high capacity for
holding water, which then evaporates and may deposit salts. A marble or
waterproof pellet will have most or all of the water run off, and leave
almost no salts to preciptate.

This may be ameliorated through the use of magnesium sulfate,
which will deal with calcium carbonate, one of the most insoluble salts,
through cation exchange and solubilizing it.

The other way of dealing with this is never letting the media go
to dryness, so salts don't "fall out" of solution (making them much harder
to re-dissolve). Of course, your plants have to be able to tolerate this
condition- not all orchids will enjoy being feet-wet all of the time.

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-AJHicks
Chandler, AZ