Well, one of the most popular choices are large cichlids, such as Oscars and Texas cichlids and the like. They are inexpensive, nice, and have loads of character. Popular, but more often then not misunderstood and wrongly kept.
Anything over 12" is too big for your tank. Even if the fish can live, anyone who keeps fish surely wants them to have room to swim and live, I would hope.
Now, with those cichlids there are two ways you can keep them. You can keep just one and that certainly means no issues, but at the same time is boring because these are social fish.
The better option is to learn how to establish a breeding pair of the cichlids in your tank. As a pair, they will share the tank and display their unique behaviours for you and even breed. This is a much more rewarding way to keep fish than one boring thing in a tank. These fish have personality, a giant catfish will just sit around. You establish a pair by starting off with extra juveniles and grow them out until a pair forms - then they will get aggressive and the remaining fish are then removed and the pair given the tank to themselves. Another bonus of this method is that you have a few extra fish in the tank while they are small to liven things up.
FYI there are no freshwater sharks - all the ones called sharks are minnows or carp and most should be kept in groups. Even the small type true marine sharks need a bigger tank than this.
