Wash fish thoroughly; pat dry.
Sprinkle with lemon juice and soy sauce.
Cut into 1 1/2-inch-wide strips.
Set aside for 15 minutes and let marinate.
Season to taste with salt and roll in cornstarch.
To prepare sauce, bring vinegar, water, sugar, soy sauce, and lemon slices to a boil over high heat.
Reduce heat to lowest point; simmer sauce for 20 minutes.
Blend cornstarch with small amount of cold water; add to sauce.
Stir until smooth and bubbly.
About 5 minutes before sauce is done, place fish in hot oil, and fry for about 8 minutes.
Fish is done when it floats to the surface.
Remove fish pieces with slotted spoon; drain on paper towels.
Arrange on serving platter and spoon sauce over fish.
Serve immediately.
Sauce can also be served separately.