Mix the baking powder, 2 cups of the rice flour and 2 teaspoons salt in a large bowl. Pour the soda water into a blender, add the frozen French fries and puree until smooth. Pour the wet mixture into the dry ingredients and stir until completely incorporated.
Pour 4 inches of the oil into a large pot with several inches of clearance. Dredge the fish in the remaining 1/2 cup of rice flour, then dip in the batter until they are well coated. Fry the battered fish in the oil, a few at a time, until they are golden brown, 6 to 8 minutes. Drain on paper towels, sprinkle with salt and serve immediately.