Eating fish that has spoiled and smells of ammonia can make you sick. The ammonia in the fish can cause gastrointestinal problems, such as nausea, vomiting, and diarrhea. In some cases, it can also lead to more serious health problems, such as kidney damage and liver failure.
If you are unsure whether or not fish is still good, it is best to err on the side of caution and not eat it. There are many other types of fish that are safe to eat and do not have the risk of being spoiled.