I had a black edged eel for a little over four years. He was 3 1/2 ft. long. For the final two years of his life, he was the only thing in my tank. He personally ate over $400 worth of fish. Once I even bought a shark, after my pet store guy assured me they wouldn't have any problems. I put the shark in the tank, he swam to the bottom, and my eel swam over to him, bit him in half, and swam away. He was extremely hardy. I couldn't kill him if I wanted to. I ended up moving though, and asked the pet store to hold him for a night or two, until I could set my tank back up. They agreed, but the first night he jumped out and died.
He was a great conversation piece though