Cut the fish into cubes. Dice the potatoes, slice onions finely, and slit chillis. Rub the fish with teaspoon turmeric and salt. Heat the oil in a frying pan and cook the fish until golden brown. Drain and set aside. Now heat a little more oil and fry the onions till golden. Add the remaining turmeric and the chilli powder, fry for three minutes. Add the potatoes and salt to taste. Fry for a minute or two before adding the water. When the potatoes are half—cooked add the chillis and fish. Cover and simmer until potatoes are cooked and the gravy thick.