Quote Originally Posted by Trevor Crone View Post
Tom, like you I'm in a hard water area (London) and now only use deionised water for making developer stock solutions and certain toners such as gold chloride. Simply to prevent precipitates forming.

For mixing working strength film developers I just use filtered tap water. However I've started to use Pyrocat-HD and I noticed a white precipitate formed when mixed with tap water. I cured this by using a good quality Brita water filter. A much cheaper option then using my deioniser, the ion exchange resin carts. are bloody expensive.
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.