In a bowl, whisk beer, flour and salt, blending well.
Rinse fish and pat dry. Cut into six oblong pieces. In a deep skillet, heat about one inch of oil to 360 degrees F. Coat fish pieces in beer batter and drain briefly. Slide fish into oil. Turn when golden. Cook another one or two minutes on the other side and remove. Drain on paper towels.
Mix the mayonnaise with a little water to thin and add the minced garlic.
To assemble the taco, add one piece of fish to each tortilla. Add a squeeze of lime juice, if desired. Top with cabbage, tomato salsa and/or mayonnaise dressing.