Cut potatoes into paper-thin slices. Combine potato and water to cover in a large bowl; set aside.
Combine flour and next 3 ingredients. Add beer, whisking until smooth. Cover and chill 1 hour, if desired.
Drain potato well. Pour oil to a depth of 1 1/2 inches into a heavy skillet; heat to 375°. Fry potato, in 4 batches, 2 to 3 minutes or until golden. Drain on paper towels; sprinkle with 1 teaspoon salt and 1/2 teaspoon pepper. Keep warm.
Sprinkle fish with remaining 1/4 teaspoon each salt and pepper. Dip fish in batter; carefully add to skillet, and fry, 2 fillets at a time, 1 1/2 to 2 minutes on each side or until golden. Serve with chips and malt vinegar.
