Use extra flour to lightly but completely coat fish. Use a freezer bag and shake or put flour on plate and dredge.
Put flour, baking of soda, into bowl or large pan. Add the beer gradually, stop when you have a thick coating type of batter.
Whisk thoroughly until it is smooth and there are no lumps.
Add the lemon juice.
Take one fillet of fish at a time and holding it by the tail or thin end, swirl it around the batter until well coated - plunge into hot fat immediately.
As soon as it has crisped up and set, add your other fillets one at a time, taking out the first ones as they cook - about 6 to 10 minutes depending on the thickness, flipping half way through. Place onto a tray and keep warm in the oven.