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.