Is that the smallest one? Ive heard they grow up to 30"long as its not super narrow a florida gar would be happy imo w/ that size ect. I would do a trio and make it a species tank or at least only select 1-2 other tank-mates ( such as bichir) and call it a nice set-up day.
Not sure what the footprint is on your tank or what kind of arowanna you have, but 150 seems a bit small to house an aro for life. If it's a silver or black, they can grow 30"+ if I'm not mistaken.I was worried about size too but I think my tank should be okay. I know I'll have some big fish in a year or so. 2 florida gars tiger oscar and 1 arowana inside a 150 gal. Let's hope I don't have to return any fish if they get too big