Mees, P 719, Mees and James, P 400 and Haist, Vol I, P 566 in which he states that fixers with Potassium salts are virtually useless. (Work of Frank and Schramm). Mees shows that K salts are roughly 4x slower as fixing agents.
So, I have found it in 3 places, besides having worked with Grant and others on some of this stuff.
Using Potassium Metaborate as a buffer in a fixer could paralyze its activity. A high percentage of potassium in Borate salts used to buffer a fixer might also slow it down or cause bad fixing.
A good stop or wash after a potassium containing developer will do a good job in preventing deterioration of the fix, but a standard test of the fixer will show either clearing time going up or silver halide being retained. Since we all check our fixer quality, this should not be a problem, right?
Patrick, you are questioning me on my home turf when you talk fixers! I learned from these people and many of the people that they refer to when I started at Kodak.