Adding new fish to a tank, whether it’s a new set-up or an existing tank, is always exciting. That is until you realize that you’ve got a bad combination of fish, which manifests itself in aggression. How does that happen?
Not every mix of species, size, and/or gender will result in tank harmony. It’s incumbent upon you, as a fish keeper, to understand the problems created when you mix the wrong fish.
First and foremost, not all species of cichlids should be kept together. Cichlids are endemic to many different aquatic environments, many with disparate water chemistry. Even some species found in the same environments don’t reside together peacefully. Also, not all cichlids are physiologically equipped to eat just anything. Some are herbivores, some are carnivores, some are omnivores.
Secondly, fish size does matter. Some cichlids are predators. Carnivorous and omnivorous fish eat other fish. Even cichlids of some species will eat its offspring or even smaller conspecifics. I once visited a pet store that had a large tank containing both adult and juvenile Tiger Oscars (Astronotus ocellatus). It wasn’t until I pointed out the rear third of a baby Oscar protruding from the mouth of an adult that the owner realized keeping them both in the same tank wasn’t a good idea. The point? Unless you’re keeping herbivores like African mbuna and similar, don’t keep small cichlids with species large enough to eat them unless you know what you’re doing.
Lastly, most cichlids are territorial. They don’t tolerate other fish within a particular boundary, including the opposite sex (unless breeding). Even courtship among males and females can have tragic results. And don’t trust that a really large tank will solve gender or territory issues. The size of the tank might only matter if the tank is really large for the species being kept and its housing only a couple of fish (and even then could be problematic depending on the species, regardless of gender).