I did not see this post before I wrote my previous one. I do not have a book by Mees with over 700 pages. There is no mention of the word "potassium" on p. 400 of Mees & James that I have. The only mention I found in regard to fixers was that a bit of potassium iodide in a fixer would speed up the wash time.
I probably would know more about these things if I made my own fixer, but I prefer to buy the best from those who do make it.
I am wondering more how potassium salts would be any more than trace amounts in borax and why they would not be given a minimum allowable number in the ANSI-PSA Standard. At any rate, it is 25 times more soluble than sodium tetraborate, and most potassium compounds are much more soluble than sodium tetraborate, so the method of fractional solution ought to minimize it quickly.