I can't amagine using a fixer on my film that has been slushing around in an open tray.
Actually, a very good point I hadn't really considered. I don't have problems with spots on negatives using the same fixer for paper and film, but I am fairly meticulous at washing everything up properly so maybe I am just a clean worker!. However, it isn't a great hardship to mix a separate bottle for film, and I think I will do so in future. Still learning!