People mix piranhas and wolves, not saying it's a good idea, if I were to try it it'd be a Mala, but I'm not into p's. I read something somewhere saying malas in the wild dig their tail in the substrate if p's show interest and defend there selves by biting back. Not sure if that's true. But I've seen the comm done maybe twice. My black is the only one of its species I've had and seems to act as the general info put out, more active then the common. Less active than say an ery. Due to being more active can be a bit of a problem on his grumpy days. My ery ery wasn't aggressive towards anyone swam around minding his business. Reminded me of my Senegal bichir only he could hover too lol. Out of all the malas I've kept, which isn't many because I've kept them for long periods of time, never had much aggression issues other than trying to assert its dominance over new comers for the first day, and then biting fish that stay directly infront of their face, but more of a warning bite never caused damage. My last Mala, which I really hope the one I have now acts like, was a puppy dog, I could pet him, pick him up a little, was never brave enough to try hand feeding cause he didn't play well with food so to speak lol, and he would hover most of the day towards the top waiting for food. Right now I have a Mala with my black both growing steadily, I feed krill, shrimp, massivore, night crawlers, and once a month a get a couple of feeders, right now their tank mates are bichirs and an odoe who will be leaving me shortly due to him becoming aggressive, the black puts him in his place but ends up with a bite or just ripped fins, the Mala and him go at it quite a bit but no one is left with marks, and now I see bites on my bichirs and they won't defend there selves, so he's gotta go, I wouldn't mind the aggression if it wasn't for the bichirs, the wolves hold their own fine. My Mala is about 7.5 inches now maybe 8 and the black is 10.5-11 inches. The odoe is around there as well.