All fish to be fried should be fileted, not whole (even the pan fish), as this allows for more seasoning to enter the meat and a more even cooking process; also one does not have to worry about encountering bones.
Most fish markets (seafood markets) do not carry "fresh" fish - fish caught that day. Fish which has been on ice for several days looses its delicate taste and picks up that "fishy" taste. Most any fresh fish has a lite succulent taste, which is lost after more than 12 hours on ice.
Choose your fish market carefully. Most don't carry a wide variety of fish (see list at the bottom). Novices (from this area) should stick to catfish or speckled trout and branch out later. Be wary of fish that have been shipped from other regions of the country --STAY WITH FISH NATIVE TO THE AREA!
If you are not a fisherman, find a friend who is and have them bring over some of their catch, so you can enjoy the wonderful taste of fresh fish.
Fresh fish is far superior to fish that has been stored on ice for several days. If in doubt, soak fish in plain milk for at least 1/2 hour and as long as overnight (in the frig). This removes much of the strong taste, making for a much milder tasting fish.
Rinse and drain filets
In a suitably sized bowl, slather mustard on filets thoroughly
Place fish fry in a large plasitc bag (gallon size). Add Tony Chacheres' seasoning if you want a little more zip. Place several filets in bag and shake well, insuring that each filet is completely coated with fish fry.
Place coated filets in hot peanut oil. Fry the filets for between 2 and 5 minutes, depending on the number and thickness of the filets and size of the fryer. It's best to use something like a large deep black-iron dutch oven in which to fry the fish, as this has a good heat capacity and will not loose temperature once the fish is placed into the oil. Some commercial fish fryers may do the job.
Be sure the temperature remains somwhat constant and high (in the 375 to 425 range). You may have to experiment to see how many filets you can add without dropping the temperature too much. If the temperature does drop, the fish will cook slowly, not producing a crispy coating-- in this case, if not cooked long enough the fish will be mushy and if overcooked, it will be too dry.
Place the fried fish in a paper-towel lined container to soak up the excess oil.
Sprinkle lightly with salt.