you get chunks of silver in the fixer after a while (from removing unnecessary silver from the negative). It forms inside the bottle and flakes into the liquid, or stays in the liquid. The silver tarnishes to dark gray/black.
You can strain it out with a coffee filter. Test your fixer to make sure it's still good before wasting time with mostly used up fixer.
If you use the same fixer for film and paper, don't. You can get paper fibers into the fixer which will get onto the film.