I think for modern films it isn't an issue. Maybe if your processing the film in warmer then normal temps then a hardner might help? I'm just using TF-2
Sodium Thiosulfate 250 g
Sodium Sulfite (anhy) 15 g
Sodium Metaborate 10 g
Distilled water to make 1000 ml
Keeps fine. At least I end up dumping it and it's still fixing pieces of film.
Which reminds me I need to make fix-))