Dry the calamari and cut into rings around ½cm thick. Lightly season with salt and pepper.
Put the flour and baking powder in a mixing bowl. Add the egg and a sprinkle of salt.
Whisk together and gradually pour in the mineral water. Add the water very slowly and whisk until there are no lumps. At this point slow down so you don't whip all of the bubbles out of the water. The resulting batter should have the consistency of a thin pancake batter.
If you're not sure of the consistency, try frying a spoonful of batter - if it spreads little and sets thick, you should think it out with a little more water.
Fry until golden brown. Place the fried calamari on a paper towel to absorb any excess fat. Plate up, seasoning with salt and drizzling with lemon juice.