just curious how big/old is the arowana now ?
I had no problems at all until my RTG hit about 14" (he's in a 240g), at that point he became much more aggressive, killing an oscar, gar and a couple large sharks he had been raised with.
Basically turned mean toward anything that wasn't a bottom dweller, the exception was my Dat. But even then he's just as nasty to the dat, the dat's just equally tough and faster so it works lol.
Since that point he's only gotten worse, killing almost anything i try to introduce within days. I at one point tried introducing a school of 6 Red Hooks silver dollars, thinning dithers would help. They were 5" ish, and the aro killed 3 the first day, actually ate 2 of them, snapped them in half to swallow them. Endlies did the same to 2 others (first time i've seen them death roll lol) and my big ornate got the 6th. Hence my suggestion to start them small. I've tried several other species and only new bichirs have been added successfully, and even then the aro will often chase them back down then they come up for air.
From what i've seen , the older the Asians get the more territorial they become, so whenever someone has a mix i ask how hold the fish is, and it seems more often then not the successful ones are smaller/younger fish, while people who have bigger adults all eventually see the mean side of them. Not always the case like any fish there are individuals who will be more or less aggressive but more often then not they turn mean with age. I also find once they get a taste for being alone or the dominate fish it makes things worse and that tanks size/length seems to help as well of course, more space allows for less territorial conflict
