This slow cooker fish chowder is made with frozen, thawed fish fillets. Make this chowder with haddock, cod, catfish, pollock, or a similar firm white fish. Serve the chowder with oyster crackers or crusty rolls. Add a tossed salad for a satisfying family lunch or dinner. Feel free to change it up with some corn for a fish and corn chowder, or add some celery. You could also add shrimp or another seafood near the end of the cooking time for a seafood chowder.