Ok, so I went and bought Kodak fixer instead of RapidFix. However, there's no mixing ratios on the package and the Kodak site doesn't help, either. Do I mix it 1:2, 1:1, or use it full strength?
Fixer in powdered crystal form? I had a box of Kodak Fixer, in crystal form, and mixed it with the amount of water indicated on the package. No further dilution required or desired. Test it by dunking a bit of leader first; time how long it takes to become clear, then fix your film for twice that time. I do that with every roll I process, just to make sure. I find liquid concentrate fixer much handier.
