This has happened to me 2-3 times before. I think it's because the bottle of fixer was left unused while it is less than full. Air gets in and oxidizes/sulfurizes the fixer inside. Those times that I was overly optimistic thinking that the fixer would still fix a roll of film properly, I wish I hadn't. Most times it required a re-fix with a fresh batch of fixer. Throwing it out is the best thing to do.