When frying any fish or seafood, fresh is always best. My rule of thumb is to eat as much of my fresh fish as possible before I store it and get it in the freezer. It's not that frozen fish is bad; not at all, your fish will just never taste as good as it does right out of the water. Of course, this is a debated topic these days with the rise of popularity in aging fish. I'll stand my ground here and say that when treated well, fresh fish is as good as it's going to get.
Now the methods you can use to fry fish are as abundant as the fish in the sea. My go-to method is a simple, light smear of yellow mustard and hot sauce, which acts as a binder, then go straight into a "breading" of cornflour, cornmeal, and seasoning. I really like this method unless I'm frying soft shell crabs, then I prefer AP flour.
Honestly, the ingredients you prefer for your batter are less important than the grease you fry your fish in. I like peanut oil, but if I had abundant access to rendered hog fat, beef tallow, or bear fat, I'd likely fry my fish in some sort of rendered animal fat.
Next, let's talk about time and temperature. The size and density of the fish you are frying will determine the time and temperature you should fry. Thicker, denser fish like wahoo might need to be fried at 350 degrees Fahrenheit for a longer time. Whole panfish like perch or sac au lait (crappie) may need to be fried at 360 to 375 degrees Fahrenheit. Shaved catfish, which is popular in Louisiana, works best fried at 375 to 400 degrees Fahrenheit to achieve a crisp chip texture.
Start with a fresh catch that you’ve bled and iced properly, batter it how you like it, use clean, hot grease, and you’ll be in the money. Once that fish is out of the grease and still sizzling, a sprinkle of sea salt and a squeeze of lemon juice goes a long way.