How long does it take a betta fish to kill each other?

It depends on the individual fish and the circumstances, but in general, it can take anywhere from a few minutes to a few days for one betta fish to kill another. If the fish are particularly aggressive, they may kill each other within a matter of minutes or hours. However, if the fish are less aggressive, they may take days or even weeks to kill each other as they continue to fight and injure each other over time. If you have multiple betta fish together, it is important to watch them closely for signs of aggression and separate them if necessary to prevent injury or death.