Timor,
Seriously, in my case it was gung in the tap water. And it is a pretty common occurence. Infrastructure and water quality varies dramatically from one location to another. Search for some recent posts about water quality. You'll find some amazing posts out there
In my case, my son did a science project analyzing our water supply for particular matter. It was a real eye-opener. That is when I bought a water filter a and a temp regulator. No more spots on my films. Or at least way less than before. I still get occasional spots when I coat my own emulsion and forget to use distilled water for soaking paper.
Eugene.
P.S. I also use tap water for mixing chemicals/washes, etc. I just use filtered tap water.