Its all about the size fo the tank,when they breed(if you have pairs) and they will! I have brasilenses and juvels,both be very agressive when breeding,the juvel are calm when not breed,but when they do, you must have a big tank if you have company in the tank,in agressive order 1.juvels 2. brasilensis(can be no 1 if they are big) 3. severum