Heat the oil in a large frying pan over a high heat. Add the seeds and cook for a few seconds until they start to pop, then add the curry leaves and cook for another few seconds until they turn golden. Reduce the heat, add the onion, ginger, garlic and chilli and cook over a medium heat for 5 minutes until softened.
Add the coriander and turmeric and cook for 30 seconds. Stir in the measurement water and tamarind and leave to simmer for 5 minutes. Add the fish and tomatoes, cover and gently cook for about 5 minutes, or until the fish is just cooked through. Pour over the coconut milk and heat through. Season and serve with plain boled rice.