There are a few individuals that are really very mean and aggressive. There is an LFS here with 25 individuals packed in a 40 gallon tank. There's an individual that has the other 24 thrusted into the corner. I took one from that crowd of timid GT's. I own one now that is not mean but is able to defend his patch of the tank.
Not particularly aggressive at all...more so than other acaras perhaps. They fit right in with other south americans like chocolates, oscars, Geo brasiliensis etc