Cut fish into 1-inch pieces. Cover and refrigerate the fish until needed. Drain pineapple, reserving juice. Set pineapple aside.
For sauce, add enough water to juice to make 1-1/3 cups. Stir in sugar, vinegar, cornstarch, soy sauce, garlic salt, and ginger.
Preheat a wok or large skillet over medium-high heat; add oil. (If necessary, add more oil during stir-frying.) Stir-fry green pepper and water chestnuts about 2 minutes or until the green pepper is nearly tender. Remove vegetables from wok. Add half of the fish to the hot wok. Stir-fry for 3 to 6 minutes or until done, being careful not to break up pieces. Gently remove fish from wok. Repeat with remaining fish, then remove it from wok.
Stir sauce, then add it to the wok. Cook and stir until thickened and bubbly. Cook and stir for 1 minute more. Return vegetables and fish to wok, then add pineapple and pimiento. Toss lightly to coat. Reduce heat. Cover and cook for 1 minute more. Serve over hot cooked rice. Makes 4 servings.