The best thing you can do for yourself is get a relatively thick piece of fish. In order to get that crisp outside, you need to be cooking at a high temperature. Usually that’s going to be in a frying pan and the frying pan surface will be maybe 300 or 400 degrees. But if you are working at that high a temperature, you’re going to be cooking the fish through pretty quickly and that’s going to toughen the inside. So what you want is a thick enough piece of fish so you can brown the outside without overcooking the inside because there’s a lot of inside to cook through.
— How to Make Sure Your Fish is Crispy, Harold McGee