Any Tempura batter recipe fits the bill. Just substitute the soda water for beer. Avoid over salting the batter as it will effect the consistency. Use a vegetable like broccoli or cauliflower to test the batter and oil temperature first. A dry rub of flour on the fish before dipping into the batter is a good trick for a consistent coating. Also make the batter on the thicker side to begin with ( about the same as pancake batter ) You can add more beer or soda water if it's too thick. Sprinkle a little salt on it after cooking it. Enjoy