It's possible that the new fish you introduced to the tank may have brought in an infection or disease, such as a bacterial or viral infection. It's also possible that the new fish were carrying parasites, such as flukes or worms, which can cause a decline in the health of your existing fish. Without knowing more about the symptoms and a proper diagnosis, it would be hard for me to give you a specific treatment. It's recommended that you consult with a veterinarian who specializes in fish or a professional aquarist for a proper diagnosis and treatment plan. Additionally, you should also quarantine new fish for at least two weeks before introducing them to your existing tank to avoid the spread of disease.

what kinds of fish were already in your tank when you added the new ones and what are the new ones? it's entirely possible that either the existing fish or the new fish were aggressive enough to terrorize the ones that died. stress can kill and it's also possible one or the other (new or old) were aggressive enough to beat up on the ones that ended up dead.