Well, because you have a 550l tank, the options aren’t limited.
Despite what people may tell you, AROWANA, REDTAIL CATS, TIGER SHOVELNOSE CATS AND ARAPAIMA ARE OUT OF THE OPTIONS!
For fish that are docile, there are a wide range of fish. Hoplo catfish, bristlenose plecos, etc.
For something that will be an exciting fish to keep, especially watch feed, exodons would be great. They feed just like pirahnas.
If you are looking for an oddball fish, Tiger knifefish would be cool. Also freshwater morays would be a great fish.
-Pic.