I used to mix powdered Ilford fixer (Ilfofix) in Melbourne tap water (fairly low in dissolved substances I think) in a 1 gallon bottle (4.5L imperial gallon). When dissolved it was very slightly turbid (extremely slightly, just a bit short of crystal clear). After a few days there was a wispy deposit and the solution above it was crystal clear. I would decant clear fixer for film use so it would be as clean as possible. Some time later, I acquired a Fisons electric still and produced my own distilled water. When I used it for the Ilfofix, the slight turbidity never settled. Same general conditions (temperature etc).
I concluded that something in the Melbourne water was assisting in settling some impurities in the powder. Photographic grade chemicals are not ultra pure, and of course fixer wouldn't be critical anyway. I have not distilled my water even for developer stock solutions for 30 years and nothing has gone wrong.