It seems entirely possible to me that Ich can come from frozen foods. Parasites can survive freezing, right? I noticed some makers of frozen fish foods advertise that they gamma irradiate the food to kill parasites.
it is pretty celar to me that when fish are super stressed from the move they get ick. My midas blenny went from a yellow to a blotchy brown during the travel from my lfs to my house and he got some ick but got over it because he ate well. None of my other fish seemed stressed at all and never got anything.
Ich is a parasite that cannot survive without a host. If your fish get Ich after being fine for a long period of time then you can suppose that some recently introduced food(?), fish, invert, sand, etc. brought the parasite in.