Clean the clams by scrubbing them thoroughly with a wire brush, removing any beard or barnacles. Then place them in a large pot with 1 gallon of water or enough to cover, and 1/3 cup salt. Allow the clams to remain for 20 minutes during which time they will expel the sand inside. Continue this process, changing the water at least two more times. The mussels are cleaned in a similar manner, but you must leave them in the water for at least 2 hours. Throw away any clams or mussels that float or are not tightly closed. Heat the olive oil in the bottom of a large, heavy saucepan over medium heat. Add the onion, tomatoes, garlic, coriander, salt, peppers, and saffron and cook, stirring, until the onions have softened, but are not brown. Add the fish and shellfish to the onion mixture. Cover with water and bring to a boil. Reduce the heat to low and simmer for 10 minutes, or until the shellfish open. Discard any clams or mussels that do not open. Serve hot.