They could be Yellow Tail Barracudas. Here is a pic of my old one:
They are far from aggressive. They are very skittish and spook easily. I believe Yellow Tails get around 12 inches. I had mine in with some cichlids, and the mix worked well because they ignored each other. The only reason I got rid of mine was because I couldn't get him off feeders. I miss him. Great fish to own.
All of mine would not pay any attention to a fish that was there same size. But if one was smaller than it was food even if it was to big for them to actually eat. I owned Ancestro falcatus which is what you have pictured sometimes called redtail barracudda.
careful if u get one my cousin got bit by his when cleanin his tank it was friggin hilarious but they are cool fish mine never bit me and they grow incredibly slow. make sure not to put it in a tank with fish that wont beat it up all the time but they are cool how they just sit there and "tick" i love em dearly but havent seen one for some while now
They do much better in small groups, even then they are very skittish. Mine also were impossible to get of live fish. If it even looked close enough to fit in their mouths they went for it. But they are not aggro, just predatory.