Whisk together flour and soda water in a medium sized bowl.
Pat fish dry with a paper towel and. Cut into smaller bite sized pieces if preferred.
Heat a shallow layer of oil in a large pan over a medium to high heat. Test the oil is hot enough by adding a small dollop of batter to the pan. It should sizzle straight away.
Dip fish fillets in the batter, allow excess batter to drip off then place into the pan. Do not overcrowd the pan. Cook in smaller batches if need be.
Cook the fish for approximately 2-3 minutes until golden and crisp then flip. Cook the other side for 2-3 minutes until crisp then remove from the pan and place on a cooling rack over a paper towel.
Serve straight away with a wedge of lemon and some tartare sauce or aioli. Chuck some oven baked wedges or chips on the side plus a salad to make homemade 'fish and chips'.