Cut the fish into pieces. Make a mixture of limejuice, little turmeric and salt with little water. Marinate the fish pieces in this mixture for 30 minutes. Then rinse well. Soak 1/2 tsp of tamarind pulp in 1/2 cup of hot water for 10 minutes. Strain and reserve the liquid. Soak the red chilies in a cup of water for 10 minutes. Strain and reserve the water. Then grind the soaked chilies, chili powder, onion, coriander seeds, cumin seeds, garlic, turmeric powder and grated coconut with remaining 1 tsp tamarind pulp to make a smooth paste. Use the chili soaked water for grinding. It is very important that the paste should be really smooth in texture for this recipe. Heat oil in a wide pan and fry the onions for 5 minutes. Add the ground paste and sauté for 5-7 minutes over medium heat. Add little water if necessary. When the oil begins to separate from the mixture add tomato, green chili and salt and mix well. Then pour 4 cups of water along with the tamarind liquid and cook for 6-7 minutes. Now add the fish pieces and cook until they are done. Garnish with cilantro leaves and serve hot. This is a very spicy dish.