Keeping cichlids is an inexact science. Just when you think you have them figured out, you realize you don't. One of the keys to understanding their behavior is to try and identify patterns. Some behaviors are typically predictable, while others are a bit more nuanced.
If you've kept both New and Old world cichlids, you've undoubtedly noticed differences in aggression. In fact, in my experience, aggression among Africans from Lake Tanganyika differs significantly from that of Lake Malawi Africans. Mbuna are generally regarded as some of the nastiest among Malawi cichlids. I call them night stalkers.
Every mbuna I have ever lost has occurred overnight, following completely normal behavior during the day before. And because there is a clearly discernible pattern here, the conclusion is that the mbuna I've had do their destruction overnight.
In my 40g breeder, I had two adult mbuna - a P. Red top Ndumbi and a P. Elongatus Chailosi. The first couple of months or so, the Ndumbi was clearly the dominant fish. Over time, however, that changed. In the last couple of months, I noticed the Ndumbi hanging in corners or within the artificial plants, with occasional chasing by the Chailosi. I watched the Ndumbi closely for any signs of damage (ripped fins, etc.) or other significant stress (not eating). Just yesterday evening, it was out in the open eating and it looked very nice. Late today, I went to feed them and the Ndumbi was dead. It had already lost all its color and was pretty rigid, meaning it had been dead a while. It had to have been beaten to death overnight.
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