my way for a good batter for deep frying,(anything!)is ,,,,
50/50% cake flour and mazina,to each 200gm, add a 1/2 teaspoon of salt, then add water to make a batter of "creamy" consistacy. leave to stand for 20 mins or so.(can use beer, bit of a waste tho!)
then you can dust the fish or whatever,with fish spice, coat in the batter and drop into hot(170c)oil.if large pieces, lower the temp to 160c or so. this batter works well for calamari(pun!), but up the temp to 180c, and fry for no more than 90 seconds!,,,,,,this is so simple,,,,but the good resterants think it`s their secret,haha. try it!