Heat oil in a large pan over medium heat. Mix in onions, garlic, crushed chillies and turmeric. Cook and stir until onions are tender, about 5 minutes. Pour water into frying pan, and mix in lemon grass and tamarind juice. Season with salt. Bring the mixture to a boil.
Stir fish into boiling mixture. Cook 10 minutes, or until fish pieces are easily flaked with a fork.