Tank mate suggestions?

I have a 14 inch black arowana along with my 3 leopoldi stingray. Only want one more fish to fill out the middle of the 400 gallon. Thinking either Korean perch or Japanese perch, possibly murray cod? No decorations in my tank, what do you guys think would compliment the tank nicely? No peacock bass plz, or emperor cichlid. Just looking for opinions, don't want to break the bank but do want something rare. Thanks.