Some of the softer foods that are being manufactured now say not to
freeze the food, but if you are using a "hard" food freezing is fine.
The reason for the freeze ban on some of the foods is for the oil
separation, as you suspect, and the fact that some of these oils break
down when frozen making them useless for fish nutrition..

Do you know any more about this? I tried googling but
I only found claims that some vege oils were not harmed by freezing.

I can't imagine a mechanism unless its a combination of seperation and
then oxidation?

Just curious...