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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).

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  • I had a similar setup (fish) in my 10-15 liters bowl some years ago, and I had absolutely no problem. Most of the fishes were mollies (whatever was "cheap", usually the black and red kinds, "all" shapes). Additionally I had a few albino corridoras, one ancistrus and one beta splendens. Is the aquarium aerated? Are the fish in the mating period? Do they have "babies"? Is there vegetation in the aquarium?
    – virolino
    Jan 15 '20 at 6:31
  • No nobö babies at that time and no mating period half the time. Vegetation: yes not a full one but 6 „islands“ of vegatation are inside @virolino
    – Thomas
    Jan 15 '20 at 6:43
  • Then you might have the situation I was told about years ago: sometimes the fish are crazy. I saw the betta attacking other fishes occasionally, and I saw (in pet shop aquariums) other fishes attacking betta. Are they fed properly (I assume yes)? Insufficient food might lead to fights, I guess. It is common in other species, including humans. Do you feed them only dry food? Maybe they lack some nutrients. Try providing them with (frozen) worms and other "natural" food and see what happens.
    – virolino
    Jan 15 '20 at 6:49
  • The 120l tank says nothing about space if you not say how many fish are in it. Jan 15 '20 at 7:35
  • @Allerleirauh about 20-30 (with 10 being ember tetra)
    – Thomas
    Jan 15 '20 at 8:00

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