I just wanted to add. When I made my new fixer up I did the usual test. The times for the old yellowed fixer was no longer then that of the fresh fixer. At least within my ability to time it. I haven't been using the old fixer for film in quite a while. Instead using it for just test prints. But it still seems to have had enough life. Well past the stated capacity and now turning yellowish. So it doesn't seem fixers need to be acidic to keep.