Your tank isn't the most friendly with "sharks" We can suggest vicious fish, however most people start with these, and tire of them because they become solo specimens or atleast very few tank mates. Make sure this is the path you want to go down. From working at a mom and pop LFS, this by far the most common problem. Unless you have multiple tanks the dropout rate of single specimen fish is pretty high within the first year.
Due to the fact that you've observed most behaivoral patterns by then. You can only watch them destroy another fish or tear apart your awesomely setup tank so many times. There are very few people who do buy a solo specimen as their first and keep it full duration.