Never reuse film fix more than 24 hours old. The little black speck on the bottom of the bottle is silver from the previous film that has precipitated out. This will stick to the next film and not even running water will wash it off. There is no home method to filter it out.
I've had this happen to me and as a result all fix goes down the drain (!) after my developing session is over. Since it can be weeks between sessions I'd rather make up a new batch of fresh fix. I don't have a lot of money to burn but a 5L jug of fix lasts a long time for me, including all the paper developing I do as well.