I ve trie dwith both fresh Ilford and Tetenal fixers at 1+4.
15 minutes might sound a bit long, yet this is how i m doing it and never had problems, until now that is. I really cant bring the temperature of all chemicals down, even the developer s temperature is increasing and i m measuring it constantly to compensate, if i see its rising fast.
When i wash, i rinse for 4 times with water, each time inverting 30 times and in the end i pour distilled water with Photo Flo, and i leave it there to settle for like 5 minutes
Now we know that you are sometimes using Ilford fixer (and we are assuming a modern form) then we can definitely say that 15 mins is far too long. If you are using fresh HYPAM or ILFORD RAPID then fix should be achieved within 2-5 mins. (as Thomas has suggested). But if you are using the powder Ilfofix II then you need 4-8 mins.
Secondly, Ilford themselves recommend this eco-efficient rinse-agitation method to wash film fixed with Ilford Rapid :
1. fill with water. 5 inversions. dump the water.
1. fill with water. 10 inversions. dump the water.
3. fill with water. 20 inversions. dump the water.
All this assumes you are not using a hardener which I would guess you're not. One last suggestion would be to follow the instructions supplied with your chemicals, and less of the improvising
btw. Has anyone even established that the extended fixing time might actually be causing the symptoms?