Go with an Odo or african pike characin there cool fish
Freshwater baracudas would be nie as we'll but need longer tanks so if you wan'ted to move them to a larger tank the i would get a few needlefish are also good choices they look alot more like gars
Bolenesox are cool fish i've been meaning to get a few they are cool fish but need to be kept with peacul tankmates smaller sized polys would work we'll the problem is they don't get very big females are said to get around 7''-8'' but males only 4'' cool fish though like a predatory molly
