Fillet the fish and cut each fillet into three, reserving all the trimmings. Put the fillets on a plate, sprinkle with salt and chill. Put the trimmings in a pan, add the water and a spiral of orange rind.
Bring to a simmer, skim, cover and cook gently for 30 minutes.
Heat the oil on high in a large flameproof casserole. Smash the garlic cloves with the flat of knife and fry until they are colored. Discard them and turn down the heat. Fry the onion gently until it is softened, adding the tomato halfway through.
Strain in the hot fish stock (and orange spiral, optional) and bring back to boil. Add the potatoes to the pan and cook for an additional 5 minutes.
Add the fish pieces to the soup, be careful not to let it boil. Cook for about 15 minutes. Add the squeezed orange juice and lemon juice, and the paprika with salt and pepper to taste.