Actually, the same thing happened to me yesterday. I poured my 1+4 dilution of the ilford fixer (that I had been using for about 1 month)into a graduate, and it was black. The last time I used it, just a week ago, it was absolutely fine. I do not use any stop bath (only water), so I could not have contaminated it with the stop bath. I use my developer as a one shot chemical, so I doubt I could have contaminated the fixer with it. Although it is summer, the chemicals seem to be only at 75 degrees when I start so I don't think it is the heat. I have been using the Ilford rapid fixer for years, but I never had this happen before. Happily, the new batch I mixed up seemed fine.