I got some free time yesterday, so I finally developed the rolls I've shot in Paris. My fixer2 was apparently used up, but my head was still mushy enough that I just noted that the films seemed strangely foggy and opaque without making the connection until I'd already developed, dried and cut them all. I did get the point eventually, so I re-fixed, washed and dried them again, and now the negatives looks fine. Not a disaster, but it did waste an hour of my bedtime. Live and learn.
In fact, one reason I didn't get it was that I don't often test my fixer. You're supposed to take a small piece of exposed, undeveloped film and see how long it takes for it to clear. If it takes more than a couple of minutes, the fixer is exhausted and it's time to mix another batch. If you develop 35mm film you always get a good-sized piece of undeveloped film leader left that you can use. But with 120-format film you don't get any extra bits to use. Now I'm thinking I should use one roll of 35mm BW film as "fix tester", and just cut off slices to test it every now and then. You only need a thin slice, so one roll would last almost forever.
#1 Cue the worlds tiniest violin playing the worlds saddest song.
#2 Film is covered with a light-sensitive silver salt. Light changes the crystal structure of the salt, and the developer converts those changed crystals to pure silver (which looks black). The fixer dissolves the remaining, unexposed, salts afterwards, leaving just the pure silver. If you don't get it all off, the remaining salts will turn a murky dark gray when exposed to light. It's not permanent (not in a short time frame) so you can re-fix the negatives, but it's a hassle and you risk scratching and damaging the negatives.