If you want horror stories...
I received this community tank from a friend that could no longer take care of them (well, the water was murky, had no filter, fish that shouldn't be in there and TOO MANY fish anyway for the tank size)
I had 4 bettas (2 males, 2 females) in the same tank (around 40-50L). No problems between them and I didn't even know they were named Siamese fighters either.
Few days later, I saw this zebra danio without 1 eye. I thought he had a disease or something. The eye was clearly missing. I didn't know what to do, just told my mom since I was a kid and had no idea. My mom had kept fish before, but in just as bad conditions as I did without filtration and stuff.
Later on, not only had that danio died, but I woke up with several more danios swimming without eyes. It was like the Zombie apocalypse in my tank!
I didn't know who to suspect, as all fish looked peaceful during the day.
Then I saw guppies with shredded tails...
Didn't know what was going on until one day, near the evening, when I saw all 4 bettas cornering a guppy male and biting its head and tail.
I got so mad at those critters that I've separated them in a 25L bowl (they had some kind of filter but it was just some sponge disk). I held a grudge against them until the end (and their end wasn't a nice one either, I had neglected their water temp and put in too warm water and they kind of got boiled).
Then there was the time I was changing water and found my past favorite swordtail male dead inside a snail shell. The stench was unbearable. X_X Since then, I haven't put in any snail shells to avoid such deaths.