Fresh white fish seems to go rather well in a turmeric and coconut milk base...it's a cooking style from the South of India.
Other things you add to bring out the flavour are ingredients like onion, ginger and red chillies. Don't forget a little salt for flavor.
If you still find you could strengthen the flavour a little -- add some saffron (you can get some for about $5 at mustafa)
If not saffron, then some coriander powder and a pinch of cumin powder and finally garnish with fresh coriander leaves.
If you want the sweet and sour element, add some sugar and a little tamarind too.