Heat oil in a deep saucepan or deep fat fryer to a medium temperature.
Whisk egg in a medium bowl and stir in sparkling water. Whisk in the flour and stir until smooth. The batter should be quite runny. Season with salt and pepper.
Dip the tuna into the batter and immediately immerse in the hot oil. Fry for about 7 minutes, until crisp and golden brown.
Place tuna tempura on kitchen paper to absorb excess oil and serve hot with fresh lemon and remoulade for dipping.