This is truly the crispiest fish batter you have ever tried. There are a couple of secrets to get that crunchy exterior and the soft inside.
Measure and whisk the dry ingredients together.
Add most of the club soda. Whisk the batter just a few times. Test the thickness. It should be fluid but thick enough to coat the back of a spoon. Add more soda, if needed. Or less soda or beer, if it is getting too thin. If it gets too thin, thicken it with a bit more flour.
Dip the thawed fillets in the batter. Let the excess drip off for a few seconds. Carefully place the fillet in very hot oil, fry until it is golden brown on the first side.
Flip and fry the other side until golden brown.