It is not recommended to put raw fish in a slow cooker. Unlike meat like beef or chicken, fish is much more delicate and cooks much faster. Slow cookers operate at low temperatures, and the extended cooking time could lead to overcooking the fish, resulting in a mushy and unappetizing texture.
Moreover, cooking fish in a slow cooker for an extended period could also pose food safety concerns. Fish can spoil faster than other meats, and the low temperatures of a slow cooker might not be sufficient to prevent bacterial growth during the cooking process.
If you want to cook fish in a slow cooker, it's best to do so with pre-cooked fish or by adding it during the last hour or so of cooking to ensure it doesn't overcook. However, for delicate fish, other cooking methods such as baking, grilling, steaming, or pan-searing are usually more suitable to maintain its texture and flavor.