Something to be careful of is that the silver can coat the inside of the bottle - you can chuck the fixer and refill with fresh, but often the flakes remain and eventually break free and contaminate the fixer, regardless of how old it is.
I got caught by this recently, dark specs stuck in the emulsion of the film due to the silver coating on the inside of the glass deciding to come off for no obvious reason into my fresh fixer....
It is a good idea, therefore, to check and clean or replace your bottle periodically.
Not a problem with the dark brown one liter glass bottles meant for chemicals I'm using, I first slosh them out with a 100 ml of new fixer, then hot water. The fixer takes care of the black flakes, and hot water readily dissolves the white powdery stuff you sometimes get floating in your fixer (sulphates?). Also being glass means you can actually check if they are dirty or not, and they are easy to clean, and the cost a couple of euros per bottle, compared to like 7-10 € for an accordion bottle.
So am I understanding the best way to keep your fixer free of the silver flakes is to run it through a few coffee filters before putting it back in the storage tank?
I have experienced this same problem. Thanks
Coffee filters worked fine for me, but I wetted the filters first, rinsing them slightly in water, because the paper can sometimes be "dusty", so I didn't want the coffee filters to leave anything behind.
Also, of the 5 filters, I used one single run and then 2 more runs with double filters.
And I poured everything into a new and clean drinking bottle, it's cheap and it's easy to check for impurities, never had a problem since.