The white precipitate from the Pyrocat series is, of course, calcium and/or magnesium carbonate. Rain water or dehumidifier water will work if it's clear. Filter out the bug parts, etc. I have high humidity in the warm months, so I have a large dehumidifier. If you used a combination of borax and NaOH or KOH to get the same pH, you could probably use the tap water.
Originally Posted by Trevor Crone