Okay, so I'm a bit of a cheapskate, and I use old fixer, but hear me out. A couple of days I went to fix some film I had just developed and went to pour up some fixer I had mixed a year or so ago, and it was full of these black flakes. I didn't think too much about it, since I use the 2-bath system, and poured it back into it's bottle and went for my second bath instead which after a clearing test proved fine so I used it. Then a day later I noticed the same black flakes in my other bottle of fixer, not as much as in the other bottle but still a bit. I decided to mix a new batch from my trusty concentrate bottle (from 2008 if I remember correctly), and fixed in that instead. I also went to buy a new bottle since the old one finally ran out. After that I also poured out the new mix into my steel-wool canister and mixed some fresh fixer. Anyhow, any ideas on what might cause these black flakes? I quite fascinated by chemistry so it would be nice to know.