Chop fish into 1 inch chunks. Season fish lightly with salt and pepper, tossing lightly to coat.
Cook Fish: Heat a non-stick wok over medium-high heat. Cook fish, in batches, for 1 minute on each side or until golden and cooked through. Transfer fish into a bowl and set aside.
Make Sauce: Add soy sauce, vinegar, pineapple, pineapple juice and tomato sauce/tomato ketchup into a large jug. Whisk to mix together.
Stir Fry Vegetables: Add onion, capsicum, zucchini, and carrot to wok on high heat with 2 tablespoons water. Cook, stirring, for 3 minutes or until vegetables are almost tender.
Combine Sauce & Vegetables & thicken sauce: Add sweet and sour sauce to vegetables on high heat and stir through until it starts to bubble. Pour in cornflour slurry (2 tablespoons cornflour and 1 tablespoon water mixed together in a small bowl). The sauce will quickly thicken on the heat. Once desired thickness is reached, remove from heat.
Add Fish back in: Add fish back into wok and stir through, letting the residual heat get into the fish pieces.
Serve: Divide between 2 or 3 bowls and serve with your favourite rice or noodles.