10-15 minutes for film in a rapid fix 1+4 is WAY too long. You are likely bleaching away some of the image.
This used to be a problem with ancient acidic fixers, but should not be an issue with modern rapid fixers.
Originally Posted by Doremus Scudder
I have used Ilford Rapid Fix at 1+19, i.e., four times more dilute than recommended, for film and have had good results; no problems with the negatives till now. However, Rudeofus has aroused my skepticism about this procedure, and I no longer use it. When I did use it, it was only for small numbers of negatives; I made sure that the amount of stock fixer was more than adequate to fix the negatives, did a clip test to determine fixing time and exceeded that time by a safety margin. I also test for adequate fixing with the ST-1 test.
As I said in that previous thread: a properly done fixer test says more than all my calculations and theories combined. My main recommendation is that one either follows manufacturer's instructions or is prepared to test ones fixer setup. These tests are simple, cheap, and easy to use.
Trying to be the best of whatever I am, even if what I am is no good.