I bought six tilapia from a local fish market, because the only other whole fish available was three times more expensive. This way, no fighting over which fish they used. With using real fish, you don't want to extend the printing to an additional class day. My pre-K through 2nd grade did this project, all doing a great job. And having a blast!
Kindergarten and up worked with a partner, which allowed us to move pretty quickly. I advise having names already on paper for the prints, which gives you assigned partners and less for the kids (or you) to do during the lesson.
All of the student work looks good, though this is one of my favorites.
A few kids were a bit squeamish about printing the fish themselves, and I heard a lot of "ews." My room was stinky and a big mess. As another classroom teacher pointed out, these kids will always remember printing fish!