I never found any problems using distilled or de-ionised water from the
local chemist or motoring supplies shop.
However I normally get mine free by talking nicely to the chemistry dept
where I work, but the 5 litre container I get filled up cost me about 3
dollars full of demineralised water, so it costs more here too.
A suitable test - for any of the silver processes is simply to add one
drop of roughly 10% silver nitrate to 5 or 10 mls in a test tube. There
should be no discernible cloudiness. Obviously this doesn't test for all
impurities, but I think you will find it ok in practice.
Water filters may not produce water that is any better than using it
straight out of the tap. Test it and see. It is a probably a good idea to
use distilled for the initial rinse with silver processes also.
Peter Marshall
On Fixing Shadows and elsewhere:
http://www.people.virginia.edu/~ds8s
Family Pictures, German Indications, London demonstrations &
The Buildings of London etc: http://www.spelthorne.ac.uk/pm/