Place the water, sugar and salt into a bowl and mix to combine. Continue stirring until the salt and sugar are dissolved.
Cut the fish into thin slices and place the slices into the bowl and completely submerge them in the brine. Place the bowl in the fridge for 5 minutes.
Line a plate with paper towels, then remove the fish from the fridge and place the slices on the plate. Pat them dry.
Cover them with plastic wrap and place the plate in the fridge for at least 4 hours.