This is exactly the same as the "Fish and Chips" recipe from Cooks Illustrated. I've simply removed the "chips" (ie, the french fries) because I never make them and it cluttered up the recipe too much.
Here's comments from my attempts:
1) Thin the batter to slightly thinner than oatmeal (maybe a thick gravy). One full beer and possibly part of another.
2) Watch it while heating the oil, it doesn't take very long to heat to 375deg, only 3-4 minutes.
3) Feel free to get the fish done ahead of the side dishes. It stays really hot for a fairly long time.
4) Two medium-size pieces of catfish cut in half is PLENTY.
5) I've started to do this with large scallops and 20-22 count shrimp. It's REALLY good.
6) I've found that the batter makes terrific onion rings.
7) After frying the fish you might try using the leftover batter to make "crunchies".
Notes from Cook's Illustrated:
Ask your fishmonger to remove the thin tail portions of the fish. For safety, use a Dutch oven with at least a 7-quart capacity. Any beer will work in this recipe, even nonalcoholic beer, with the exception of dark stouts and ales. Serve with traditional malt vinegar or with tartar sauce.
3 quarts peanut oil (or canola oil)
1 1/2 cups all-purpose flour
1/2 cup cornstarch
1/2 teaspoon cayenne pepper
1/2 teaspoon paprika
1/8 teaspoon ground black pepper
1 teaspoon baking powder
1 1/2 pounds 1-inch-thick cod fillet (or other thick white fish, such as hake or haddock) cut into eight 3-ounce pieces
1 1/2 cups beer (12 ounces), cold
1. Whisk flour, cornstarch, cayenne, paprika, pepper, and 2 teaspoons salt in large mixing bowl; transfer 3/4 cup of mixture to rimmed baking sheet. Add baking powder to bowl and whisk to combine.
2. In heavy-bottomed Dutch oven, heat oil over medium heat to 375 degrees. Meanwhile, thoroughly dry fish with paper towels and dredge each piece in flour mixture on baking sheet; transfer pieces to wire rack, shaking off excess flour. Add 1 1/4 cups beer to flour mixture in mixing bowl and stir until mixture is just combined (batter will be lumpy). Add remaining beer as needed, 1 tablespoon at a time, whisking after each addition, until batter falls from whisk in thin, steady stream and leaves faint trail across surface of batter. Using tongs, dip 1 piece fish in batter and let excess run off, shaking gently. Place battered fish back onto baking sheet with flour mixture and turn to coat both sides. Repeat with remaining fish, keeping pieces in single layer on baking sheet.
3. When oil reaches 375 degrees, increase heat to high and add battered fish to oil with tongs, gently shaking off excess flour. Fry, stirring occasionally, until golden brown, 7 to 8 minutes. Transfer fish to thick paper bag or paper towels to drain. Serve.