Clean fish and make vertical cuts across fish to facilitate marinating.
Marinate fish with salt, turmeric and corn starch. Leave to marinate for at least 30 minutes.
In wok with 2-3 cups of oil very hot oil (>=350 deg F), fry fish for 2-3 minutes on each side. Take fish out and put into serving dish.
In the same wok with 2 tbsp oil, stir fry onion, garlic, ginger for 2-3 minutes.
Add ketchup, oyster sauce, fish sauce, sugar. Stir and bring to boil.
Add zucchini. spring onions, can diced tomatoes and green chilies.
Bring to boil and simmer for 5 minutes.
Pour sauce over the fried fish