Put flour, seasoning salt, garlic powder and paprika into a large bowl and whisk to combine.
Add beer and 1/4 cup water and whisk to make a smooth batter.
Pour the oil into a large heavy pot to a depth of 2" and heat over medium high heat until temperature registers 350 F on a deep fry thermometer.
Working in batches, dust the fish with cornstarch lightly, dip the fish into the batter, shaking off some of the excess, and fry, turning occasionally, until cooked through and deep golden brown, about 5 minutes. Transfer the fried fish to a rack to drain.