Quote Originally Posted by Photo Engineer
Properly compounded C41 fix is pH ~6.5. The correct range is about 6.3 - 6.8 for proper activity. (20 deg C)

There are two forms. C41 and the RA version which contains Ammonium Thiocyanate. It is much faster than the version without SCN.

PE
The one with thiocyanate is much more expensive, I think (maybe 3x the price).