I've got a recurring problem with killer fish. In the last 3 years I repeatedly had recurring sudden spikes in fish deaths.
Via pure coincidence I found out that it was no strange sickness or water values, but instead fish killing one another (I saw one of the cases). The "murderers" were mostly sword bearers and black mollies. Strangely the females if I'm not mistaken (sadly I watched it only through the corner of my eyes so not 100% sure).
The things they killed were members of other species like a black molly killing a sword bearer and vice versa. Also other fish types were victims.
What is strange to me is....space shouldn't be an issue as it is a 31,7 gallon (120l) tank with a few caves and quite a few plants.
For the "killing method" it was so that the murderer bit into the forehead of the other fish, which then panicked, and swam upward and away with all speed....the speed being so high that it jumped out of the water and hit the lid of the aquarium with full force....the result for a human would have been a neck breaking equivalent from what I saw. The fish then sank down and shuddered for a few moments before lying still and being dead.
So far it has been about 3 fish that went on killing sprees... each killing 2-6 others over the course of weeks. That is before 1-2 months after the deaths started....they died themselves and the death rate sank to 0 again.
So far I found nothing indicating any reason for this behaviour (only found a bit about males and space, but at least 1 was very probably a female).
My question is: Are there any other possibilities on what I'm doing wrong there....so that my fish become so overly aggressive?
(Aside from the killing sprees I saw no strange behaviour for any of the species or involved specimen).