I have this four year old Kodafix. One bottle opened. The other sealed. Got them out the other day and notice that both have a pale film settled onto the bottomward surfaces (bottom of bottle and ridges in sides). I presume it is time to replace it? Shaking on and gently agitating the other did not seem to dislodge any of the whitish matter or cause it to mix back into solution.
I would filter it before use, and not get any of that residue into my final fix. It could lodge in the film and leave spots.
Also, there is a better chance of getting an answer in the thread "Ask Chris". It has a plethora of good advice from this dude who knows just about every answer to every question!
Fixer does go bad. The whitish sediment is probably sulfur. The particle size can be very small and difficult to filter out. Coffee filters will not do the job as they are designed for coarse particles. Fixer is cheap I would suggest dumping it. Should any of the particles stick to film emulsion it is impossible to remove them. Additionally once a fixing bath starts to go bad the problem continues at a faster and faster rate.
Knowing that fixer goes bad should influence just how much you purchase at a time. It makes no sense to try and stock-pile it.
As PE points out coffee filters can vary in their effectiveness. Some can be very coarse. Usually you get what you pay for. The very best that I have found are those made for the Chemex coffee system. They are made from lab grade filter paper.