Thanks P.E. I'd never thought of that. Shoulda listened in high school chem classes! I've noticed the keeping properties as well. I tend to buy mini-lab C-41 fixer and use that. The container just lists ammonium thiosulphate - shouldn't be a problem; should it?
It depends on which C-41 fixer you have. The RA Flexicolor fix has more ingredients than the plain C-41 fixer, but they both contain sulfites, EDTA or some sequestrant, and buffers. So it is not a simple solution of Ammonium Hypo.