It may not have been properly fixed in the first place. you can try and refix it, but the color changes may still remain. This happens every once in awhile, where a student of mine rushes the chemical processes by counting too fast. When I hit the lights to evaluate them, you see them change into a purplish color quickly, which at this point i turn off the lights quickly, and drop them back in the fixer. it gets rid of a good deal of that color shift, but areas still somtimes retain a yellowing.
The solution is simple; your fixer is too cold. That's why your student turns purple when you drop him/her into it in the dark.