I've read about the clearing time test for old fix, but I need to have some details clarified. The test is simply to drop a strip of undeveloped film into the fixer, and see how long it takes to clear completely. Apparently, my fixing time should then be at least twice that time. Now, do I agitate the film when I do this test, or do I leave it alone? I would expect agitation to speed up the clearing time of a piece of film compared to just resting on the bottom of a tray or beaker. How rigid is that factor 2? Do different films need different fixing times, or are they all the same?