Sift the flour and salt together into a bowl. Add the eggs and half the soda or mineral water, and whisk together until smooth. Gradually whisk in the remaining soda water until the batter is smooth.
Fill a deep heavy saucepan about one-third full with oil. Heat the oil until a cube of bread browns in 30 seconds.
Dip the squid rings into the batter, a few at a time. Carefully drop into the hot oil. Fry for 1–2 minutes until crisp and golden. Drain on kitchen paper, and serve immediately.