Yes they are hostile imagine a 5 or 6 in male(they do get this big if they don't breed) holding his own against 10 -12 in. amphilphos & veja. species. They are one of my top 5 five killers,fish I've owned that destroyed other fish I've owned. I guess the dull (except when breeding) color is why many people don't keep them. thier cousins neet. panamensis aren't as aggressive but are prettier coloration if you want to try them. Neets. are one of the only fish I used to keep with much larger non-parachromis species regularly. I had a 4 in female dogg a 5 in salvini & goe. steindachneri until one jumped out the tank the other had to be removed.