Look at the bottom of your fix bottle. Silver precipitates out from used fix starting in a day or so. They appear mostly as flakes , not round spots although some are round. They stick to the next film like glue, more to the denser area, but can be abraded off carefully if film is still wet. Once dry , forget it.
I have never found a home filtering procedure to remove the silver perfectly. Bounty brand towels quartered in a funnel will filter off the bigger pieces. MUST be Bounty, nothing else.
I never reuse fix on important film and I do filter and use it up on test prints.
Get a glass bottle and clean it out and use it for fix.