Cats should not eat raw fish because it can cause parasites, leading to jaundice and hepatitis. If a cat already has liver disease, eating raw fish can also cause ascites and kidney disease. Furthermore, it can cause a deficiency in vitamin B1, as raw fish produces an enzyme that destroys vitamin B1 in a cat's body, leading to seizures, heart disease, intermittent shock, and other problems. 
Precautions for cats eating fish
1. Beware of fish bones. Fish bones are quite hard, and when a cat chews them, they can produce sharp fragments that can sometimes injure a cat's mouth or internal organs. Therefore, you should prevent your cat from eating these bones unless you use a pressure cooker to soften or crush the bones before adding them to its food.
2. Limit dried fish: Dried fish contains a relatively high amount of magnesium, which can easily induce and cause urinary tract stones or urinary system diseases in cats, so try to limit their intake. An adult cat needs about 0.3 to 0.5 grams of salt per day, so be careful not to add too much salt to their food.
3. Avoid seafood. Squid, cuttlefish, and some shellfish contain components that cats may not tolerate well. Excessive consumption can cause indigestion and gastrointestinal problems. Dried squid absorbs water and expands in a cat's stomach, so it should not be given in large quantities. Some seafood can also cause skin inflammation in cats. Before feeding them seafood, give them a small amount first; if there is no adverse reaction, then you can give them a larger amount.
