In a shallow bowl, combine the potato flakes and rice flour. In another shallow bowl, lightly beat the egg with 2 tablespoons of water. Season the cod fillets with salt and pepper. Dip the tops into the egg mixture, then press into the potato flakes. Put the remaining 1/4 cup oil in a large unheated nonstick skillet. Add the fish, potato sides down, and cook over high heat until golden, about 7 minutes. Carefully turn the fish, lower the heat to moderate and cook until the flesh flakes easily but is still slightly opaque in the center, about 3 minutes longer.