For things like Piranha, give it to someone where it is legal to have them.
But, say you are at a pet store, and see a nice looking bowfin. You buy it, and after a few months you realize it's actually a snakehead. If you were to report yourself to authorities that you owned this snakehead, and asked what to do with it, would you still get in trouble? Its not like you were trying to hide the fact you had it, and when you discovered what it was, you would be doing the right thing, right?
Or would they slap a fine on you for having it? Or jail you? Hell, would there be a way you could get a permit to keep it?