This may sound like a stupid tip. However here goes.
The water supply where I live has quite a high lime content so will leave marks on any film that I process if I am not careful. I do have access to a regular supply of 'distilled water' but from an unusual source - namely the condensing clothes tumble drier. This water is condensed out of the drying clothes and is collected in a storage container fitted to the drier. When full it holds about 6-7 pints. Up until now has been simply poured away but last night I saved some and checked it for content of any particles, hairs fibres etc and then filtered it and - nothing. I processed a B&W film this morning and did a final rinse in the saved water and it dried perfectly with virtually no marks at all.
Just curious, why is the moisture condensate collected by these dryers rather than just vented outside?