EVERY chicken, though? I could agree with that if it was only one or two of them, but it's all eight, and they're varying breeds. And, I'll say once again, the homemade mix they were on before contained flax seeds, which made up about 12% of the feed.
All it take is one bad egg and the rest will take on the smell.
Eggshells are not solid or airtight, air can move freely in and out of a egg and so can odors.
If you have one hen that lays a "fishy" smelling egg do to genetic defect and you keep all your eggs together then all the other eggs can pick up the smell of that one egg.
When you said that the smell was worse when you fed a fermented feed that told me that it is very possible that one of your birds has the genetic defect that causes her eggs to have a fishy smell.