There is no definitive answer to this question as it depends on a number of factors, including the species of fish, the size of the fish, and the environment in which it is kept. However, some experts believe that certain species of fish are more difficult to keep alive than others, and that certain environments may be more challenging for fish to survive in.
Some of the hardest pet fish to kill include those that are native to fast-moving rivers or streams, as they are accustomed to high levels of oxygen in their water and can often withstand poor water quality. Fish that live in slow-moving or stagnant waters may be more sensitive to changes in their environment and may be more difficult to keep alive. Additionally, larger fish generally require more care than smallerfish, as they need more space and may be more susceptibleto disease.