Controlling Aggression Through Stocking -
Second, we need to keep in mind that Cichlids which have a similar outward appearance are more likely to be the focus of attack. The more similar, the more likely it is a competitor. You should try to avoid keeping fishes with the same coloration or body shape. This can be helped by keeping a diverse collection of Cichlids. In other words, you don't put 5 species of Aulonocara (Peacocks) into a 55 gallon tank, but instead you would choose 1 species from each compatible genus. This is also a benefit if you are trying to breed your Cichlids, especially if you are contemplating selling your offspring, as fishes of the same genus will often interbreed producing unwanted and unattractive hybrids.
Related to the statement above, males are more likely to attack males. This is simply avoided by not keeping many males in the tank. This technique is especially important in smaller tanks and with polygamous fish. This is why we often recommend to keep pairs, trios, or even more, of these fish, but only one male per species. This also cuts down on the aggression towards females. In some instances, pairs are not a good idea, as a male which is healthy will constantly court and chase a female. By keeping more females than males, no single female takes the brunt of a male's aggression. In some cases, you may end up with a tank full of several gray females and only a few colorful fish, but, in the long run, the community will work better, and the fish will be in better condition (3 males beating on each other until 2 die only produces a rather mauled victor). There are a few exceptions to this "rule", but that's exactly what they are, exceptions. In most instances, we also refrain from keeping tanks with only males (although they can be very striking displays) because they are infinitely more hostile environments. In addition, some male Cichlids will not display full coloration when not kept in the presence of females. However, when you are dealing with someone who has a knack for doing just that, it may be the way for you to go.