
In case it helps:
"Tom is chopping heads OF fish." - Here the "of" links the words "fish" and "heads" together. "Heads of fish" means the same as "fish heads".
So in this case the whole sentence means that Tom has fish heads, and he is chopping the heads into smaller pieces.
"Tom is chopping heads OFF fish." - Here the "off" belongs to the word "chopping", and it means that a part is removed from the whole. (just like in "cutting off")
In this case the whole sentence means that Tom has whole fish (not just heads) and he is chopping the heads off of the fish, so he gets fish without heads.