I found the same method, living in the Desert our water is very very hard, at time I have left out a glass overnight and found a thin white film in the glass the next morning. I now mix all my chemisty paper and film with distilled water and only use tap water for rinse. Although I have not kept tack of the number of prints to a gallon of fix it seems to me that my fixer is lasting longer when mixed with distilled water.