Is it ok to use "onto" instead of "into" here? What is the difference in nuance between those two?

Native speaker’s answer
Rebecca
You could use "onto" here! The context would allow for it. Here, the difference between "into" and "onto" is that "into" emphasizes all the little parts of the fish that the paper could go in to make a more accurate mark. Placing the paper "onto" the fish would mean just placing it on the surface of the fish. But in general, "into" means, something goes "in" something in some way, "onto" implies the direction of going "on" something. Ex: Put the oranges into the bag, please, Rachel. Ex: Peter put his bag onto the table.