Dissolve salt in water. Make 3 slashes in fish and soak in salted water for 15 minutes. Drain. In food processor, blend shallots, garlic, red pepper, turmeric, ginger, and coriander to form a paste. Mix this with coconut milk, brown sugar, galangal, lemon grass and tamarind liquid. In a saucepan, bring the mixture to a boil and cook over moderate heat for 10 minutes or until about half of the liquid is evaporated. Dip fish in coconut sauce and grill under a broiler for 10 minutes on each side, basting generously. Serve with rice.