So i bought a random silver and orange fish before researching or knowing what it was, i know dumb and dangerous for the fish, i dunno i was excited because ive only had one oscar in a 150 gallon for a loong time. the pet store guy had a thick german accent so im not sure exactly what he said (i asked him a few times to repeat but still couldnt make it out), im thinking its a red bellied pacu
or piranha.

soo now a month later im sitting here realizing i need to know about this fish. and finally getting internet access here i am.

firstly i bought this fish and it was tiny, now its even bigger than my oscar! how do i tell the difference b/t piranha and pacu? how big do they get?
and the pet store guy (prolly taking me for what im worth) told me my 6 in. oscar and the pacu would get a long fine, which they have been since day one. But now that the silver guy is getting bigger, and my oscar getting for some reason slower, im worried the new fish is gonna punk my oscar, he hasnt yet but hes now ever so slightly bigger and kinda goes all crazy hitting tank walls and making my oscar swim away from him.

also there is a goldfish that somehow got away from both fish and is pretty big now, about 3.5 inches long. i like ‘im now, will he eventually be eaten or starved?

i dunno please help, i need to differentiate bt the 2 fish, i think its a pacu cause its not really aggressive, please give me some general caretaking info and specs.??
thanks!