In a large bowl, whisk together the flour, baking powder, garlic powder, paprika, salt, and black pepper. Gradually add the beer, stirring until the batter is smooth and well combined. Let the batter rest for about 15 minutes.
While the batter is resting, heat about 2 inches of vegetable oil in a deep skillet or pot over medium-high heat until it reaches 350°F.
Pat the fish fillets dry with paper towels and cut them into strips or bite-sized pieces.
Dip each piece of fish into the batter, allowing any excess to drip off, then carefully place it in the hot oil. Fry the fish in batches for about 4-5 minutes or until golden brown and crispy, turning occasionally. Remove the fish and place it on a paper towel-lined plate to drain excess oil.
Warm the corn tortillas in a dry skillet over medium heat for about 30 seconds on each side.
To assemble the tacos, place a few pieces of fried fish on each tortilla, top with shredded cabbage, diced tomatoes, and cilantro. Serve with lime wedges and a dollop of sour cream or your favorite sauce, if desired.