The first point is to ensure adequate populations, with many species of fish. It is quite important to have enough fish in the aquarium. It is important to have the right populations of fish, because in the wild, particularly your common cichlids (natively known as mbuna, meaning `rockfish`) are basically a fish which is found amongst the rocks, and the environment that they come from is very busy. There are a lot of fish around, therefore, if you get these species of fish and put them in an aquarium with too few fish, then it makes them stressed, because they are wondering where everybody is. This is because in their natural habitat mbuna will be swimming around through the rocks, and then above them are another species of cichlid called the utaka. The utaka are schooling fish that swim in the mid-water. When the utaka are present, the mbunas, are very comfortable and are quite relaxed, swim around, and their stress levels are quite low. But then as soon as the utaka take off, the mbunas get extremely stressed and will hide in the rocks, becoming quite aggressive.
The reason why the utaka will usually take off is because the lake predators are there, such as the lake clarifers. In Africa where these fish come from there are a few different types of predators, so when the schooling fishes take off, the mbuna are going to hide amongst the rocks. This is why you will often have an aquarium which is a lovely, big aquarium, but all your fish are just hiding in the rocks. They just don’t want to come out unless it is feeding time during which they will eat some food and then swim back into the rocks again. Their stress levels are actually quite high because, naturally, they expect to have a lot more fish around. Having more fish around makes them feel more comfortable.
If they feel comfortable, then they stress less; if they stress less, they are healthier; and if they are healthier, they tend to be more relaxed and less aggressive. Having these species of fish, if you have less fish, they are more aggressive because they get a lot more territorial. They are looking to keep their favorite holes because they have no threat, like an inbuilt threat of predators. So, with some species of fish, such as cichlids, having plenty of fish in the aquarium is very important.