In deep fat fryer or 4-quart Dutch oven, heat 1 1/2 inches oil to 350°F. Cut each tilapia fillet into 8 rectangular pieces, approximately 4x1-inch each.
In large bowl, mix club soda, flour and taco seasoning mix. Dip tilapia pieces into batter, and carefully transfer to hot oil 6 pieces at a time. Fry about 4 minutes, turning once, until golden brown (tail sections may cook more quickly). Remove with slotted spoon; drain on paper towels.
In large bowl, mix cabbage, sour cream and salsa. Spread inside tops and bottoms of buns with refried beans. Fill each bun with 3 or 4 pieces fried tilapia and cabbage mixture. Serve warm.