Have an aggressive cichlid or a group of cichlids and aren’t sure how to mitigate the behavior? Look at what you can control within their environment.
It is no secret that one way to distribute aggression amongst a tank of mbuna is to overcrowd them so no single fish can establish a territory. This is often the reason you see tanks full of these colorful, but mildly tempered cichlids from Lake Malawi.
Breaking up sight lines is another strategy. This is especially useful for more benthic species like shell dwellers. In other words, introducing objects that limit the vision between fish across the tank, like plants, rocks, and other items, has proven to reduce territorial aggression.
Often overlooked is another option that is as simple but potentially more effective. Reduce the water temperature. As water temperatures go down, so does the metabolism of tropical fish. One result is that there is less energy to expend on both defensive and offensive aggression. Just a simple temperature reduction of 2-3 degrees may be enough.
There are many studies to date that discuss this in more detail, some that specifically correlate elevated aggression with elevated temperatures.
Below are two more recent cichlid-specific studies that are worth your time, if you’re interested in learning more:
Reference: Jones, Nick A.R., Newon-Youens, Jade, Frommen, Joachim G.Takahashi, T. “Rise and fall: increasing temperatures have nonlinear effects on aggression in a tropical fish.” Animal Behavior (2018). https://doi.org/10.1016/j.anbehav.2023.10.008.
Ribeiro, Maiko Willas Soares, Oliveira, Adriano Teixeira, Carvalho, Thais Billalba. “Water temperature modulates social behavior of ornamental cichlid (Pterophyllum scalare) in an artificial system.” Journal of Applied Aquaculture (2021). https://doi.org/10.1080/10454438.2021.1973936