I would probably get a Hujeta Pike characin (Ctenolucius hujeta), commonly sold as Rocket Gars, Silver Gars, or Freshwater Barracudas. They don't grow to be very large (8-10inches), they're peaceful with anything too large to eat, but when it's feeding time they are very fast and fun to watch! They can also live in Brackish water so if you decided to have a brackish tank, you could have one Hujeta pike, a couple spotted, figure eight, or ceylon puffer fish, and a violet dragon goby (Very cool looking eel-like fish). The dragon goby may eventually outgrow the tank, though and need a 55 gal. Hope this information helps!