^^ a school of exodon would be awesome! Ctenopoma are a pretty cool small predator.
I was suprised no one said it until now. I was thinking it the whole time I read this. Ctenopoma, or leopard bush fish also called a African leaf fish but there is actually another african leaf fish that more resembles a leaf. Ctenopoma are ambush predators and like to camouflage into the plants and wood in the tank. Pretty easy to find as I got mine from petsmart at about the size of a quarter for $8. Hes a great fish.
Someone else said butterfly fish also a very cool little predator. Just need a top to the tank and you'll need to keep the water level low enough as they typically stay at the top. Mine eats krill from my fingers. Its like a small long fin arowana. I had 3 which were living together for a while and suddenly 2 ganged up on the one and it had to be rehomed. So I dont know how well they can be kept in groups. The 2 would fight from time to time also.
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Chicxulub
said the wolf fish looks pretty good and from everything ive seen or read they are pretty fierce. I've never owned one. I think you need to be careful in which ones you get as I think some get really big.
Also stated above is one of my favorites polys! Polypturus or Bichirs are a great fish, ancient predators. You could get a couple senagals in that tank no problem they are cute and fun to watch. Sens would be the limit on what kind you could get because of tank size. Maybe a polli but they say those get 12" while ive had mine 2 years + and she is only 6ish". Also need a lid.
I'd go out on a limb and say you need a lid with any fish but probably with all the fish we have listed here especially.
Last but certainly not least Welcome to the forums!
I'm not gonna lecture you just know there has been in influx of google know it alls lately. Watch out for them like said above look for medals next to the name look, at fish type under the name the bigger the fish then more posts and time here. On that, time here doesnt matter so much but time in the hobby is vital for hands on experiance.
Good luck and have fun.