Apparently, you've discovered that you have either under-fixed your film or that your fixer was exhausted. You can prevent this from happening again very easily. Test your fixer every other time you use it by fixing out a small undeveloped piece of film which has been soaked in water for a few minutes. This is an excellent use for all those 35mm film leaders that get clipped before loading a reel. If you don't shoot 35mm, just buy a roll of the cheapest B&W film you can find. A 36 exposure roll can last a long long time since you only need 1/2 in or so each time. Note how long it takes for the test strip to clear completely. Then double that time for conventional films and triple it for T-grain films. When the clearing time exceeds 3 minutes, the fixer is exhausted enough to discard.