What to Do with This FISH !

Fish stores will often take a donated fish and put it in the aquarium with other similar fish. Sounds like you got some bad advice about "community" fish vs. aggressive fish. In the short run, you can isolate one part of the tank from the other with a mesh screen - water and oxygen flow easily but no more death and destruction!

You have to research which fish you can actually pair together. You also have to research how many fish per gallon you can safely home in a tank. Stop replacing fish until you've done your research.

Stop replacing the fish he kills and just keep him until he dies of old age.

Talk to the people at the fish store.
Whenever you own pets (even bottom of the food chain pets like feeder fish) it's a good, and humane, idea to know exactly what you're dealing with.

Cage Match! Buy another aggressive fish and let them fight to the death. Juuuust kidding.

I would ask the pet store if there are other types of fish that he can coexist with peacefully and put them in a seperate tank.

What kind of fish is it? That would be helpful to know.
If it happens to be a betta, they are very aggressive, but there ARE fish that can live with them... ask at the pet store.

I think you need to find out what kind of fish it is so you can determine what type/size of fish it will be compatible. It may very well need to be the smallest fish in the tank.

Ask a pet store what fish would be compatible with him, find a new home for him (Not in the drain), or just keep him alone in the tank until he passes on.

When I was a kid, we had a friend give us a gorgeous fish tank as he upgraded. He only gave us one fish.

Well, feeling he might be lonely, we got a bunch of other fish. The next morning, there wasn't another fish in the tank.

He was a Jack Dempsey. He ate all the other fish. It was a little detail our friend didn't tell us.

See if you can sell him to a pet store or a private owner. The one fish will have to go unless you intend to continue to purchase it "feeder" fish.

Best wishes.

I think if you're going to have pets you have to do your research... and I do consider fish pets. It would be helpful to know what kind of fish it is to be able to give any kind of advice about keeping him. But I am not an advocate for flushing a fish just because you don't particularly like it. Some fish can not live with other fish easily because they are aggressive and carnivorous, but it isn't to say that it must live by itself. For instance goldfish are known to be mostly able to only live with other goldfish... anything else that is small enough to fit it it's mouth it will eat.

Stop replacing fish and do some homework, or give it to someone who will care for it properly.

After reading your SWH, do the research yourself. Who ever you're talking to doesn't know much about fish if they told you two rainbow sharks could live in the same tank. They are aggressive fish toward each other. You don't mention how big the tank is, but these fish need room to swim and to hide. If it's less than 30 gallons it is sure to become more aggressive. Releasing it into the lake will be just as bad as flushing it imo... it's not likely to survive there.

Bag him up, and give him to a pet store!

Take him back to the fish store and ask to exchange him for another who isn't so aggressive :)

The store where we buy our fish lets us trade in fish. I would find a place that does this.