Many years ago (in fact many more years ago than I care to recall in numerical terms), an old timer who ran a studio in my home town, taught me a simple trick toimprove the life of fixer. He said he got it from old Kodak literature, by this he most likely meant 1920s or 1930s.
Adding 25 ml of fresh Kodak fixer (from a new bottle) to a mixed batch of older fixer (by this I mean from a long outdated bottle).
Adding 1-2 grams of sodium sulfite to the old-new fixer mix. Premix the sulfite in hot water and add it to the mix. Be sure to count the additional liquid as part of the final total mix, eg 250 ml, 500 ml or 1000 ml, whatever you are mixing up as your final mix.
Filter several times. I use Melita coffee filters with a clump of cotton wool on the bottom. The guck this picks up (and removes from the fix mix) never ceases to amaze.
Don't overuse the old-new fixer mix. Turf it after a reasonable number of films. Best to mix it in small batches (I do 250 mls at a time), use it for a few rolls, and then throw it out.
This has worked for me for about, say, 40 years. Visual inspection of my negatives from the '70s and '80s show no deterioration due to bad fixing.
Of course I can make no scientific claims for these recommendations. If others with more knowledge of photo chemistry than I have, care to comment, I will be very interested.