There's some strategy to making good seafood soup: Use good-quality fish or shellfish, add the ingredients in stages, and don't overcook.
Fresh fish and shellfish should be mild-smelling, with no fishy odor or trace of ammonia, and the flesh should feel firm. If you buy previously frozen seafood, be sure it hasn't been defrosted longer than two days.
Because a soup made with fish or seafood combines ingredients with varying cooking times, you'll need to add the different items in stages. Potatoes, onions and other sturdy, aromatic vegetables go into the kettle at the beginning, whereas any green vegetables and the fish or shellfish generally get added toward the end of the cooking. It's a rare seafood soup or stew that needs more than half an hour's simmering after you've added the fish or shellfish. The reason? The delicate proteins in fish and shellfish don't stand up well to prolonged cooking.