Heat oil in a deep-fryer or large saucepan to 350 °F.
Stir batter mix and beer together in a bowl.
Dry cod pieces thoroughly before putting them into the batter.
Dip cod in the batter mix; fry coated cod in batches in the hot oil until cod is cooked through and the coating is brown, 4 to 5 minutes. Do not overcrowd the pan.
Remove cod with a slotted spoon and drain on a paper towel-lined plate.
To serve, lay tortilla flat on a plate, top with taco slaw, 2-3 pieces of fried fish, a sprinkle of cheese, and fresh salsa.
Drizzle with fish taco sauce, chile-garlic sauce, and lime juice if desired.
Repeat with tortillas.