Pat fish filets dry with a paper towel. Cut in half horizontally if desired, or leave whole.
Heat 2 - 3 inches of oil in a large, heavy bottomed pot over medium high heat to 375 ℉.
While oil is heating, sprinkle fish filets with ¼ cup of rice flour and a pinch of salt, coating both sides.
Whisk together flour, rice flour, baking powder, and salt. Just before cooking, remove beef from fridge and pour a cup into the flour mixture. Whisk until incorporated. Don't over mix. The batter should be pretty thin, but able to coat the back of a spoon.
Carefully dunk a filet into the batter, making sure all sides get coated. Allow excess to drip off.
Carefully lower into oil. Don't overcrowd, so depending upon the size of your pot you can do one or two at a time. Fry for 3 minutes, flipping after 2 minutes, until golden brown.
Remove from oil and place on a rack to drain and cool. Repeat with remaining filets.
To serve, spread tartar sauce on both buns. Top with a piece of fish, lettuce, and tomato.