I've had 2 uaru's (10 inch or so) which were the most agressive fish i've ever owned. Worse then fish like Jaguars, Kelberi, Festae, Rivelatus, Crenicichla Atabapo, Nandopis Haitiensis I've kept. And even worse then the terror pack of Chiapaheros Grammodes I had.
They killed a tiger shovelnose, a petenia and a hoplarchus psittacus. And would have killed a lot more fish if i hadn't separated them. Absolutely crazy territorial and if they felt like it they would go through the entire tank to find a victim to chase and bully.
from all i've heard this was an exception for Uaru's. But for me: never again