as for as achara's being "non-predatory" I had given the tank a dozen gold fish as treats for the gar, payara, and ck but the achara had came out and hunted down about 4 of them. it was pretty neat to see.
Do you keep him well fed? It could be that he was really hungry and/or the herd instinct. He coud be "special" tooJust because they are scavengers in the wild, where it is not easy to catch a live meal, in the confines of a tank it all may change. I think if goldfish were a bit larger and had some defense in the form of spines, like e.g. catfish and clown loaches have, they'd last.