Omgoodness the other day we had a fish that was getting beat up so we put him in the other tank we had that had guppies in it well he ate the guppies and was working on two more fish so we took him out of that tank and put him into a tank by himself and he started getting cold and acting weird so we got a bag for him to get used to the water so he wouldn't get shocked when he got to warm in the other tank. well he jumped out of that bag into the tank water and got shocked by the water and he died unfortunately .
Buy another aggressive fish to put in the tank. You have to find the right balance. My inlaws have a huge aquarium of semi aggressive fresh water fish. On numerous occasions when we have visited we have bought our own fish to put in. Starting with small cheap fish, working our way up to bigger semi aggressive fish. The longest one lasted before their fish killed it was 6 months. Get the right fish together and they can last a very long time.