Almost inevitably any hobbyist who keeps cichlids will want to mix species. Fish of different origin, size, and aggression can be kept together in harmony if a few things are kept in check. The first rule of mixing aggressive cichlids you must understand is that nothing is set in stone. Each individual cichlid has a varying character and therefore no aggressive cichlid community will be exactly the same.
There are two major factors in determining the number of fish that you can provide for properly in an aquarium. Aggression and Bioload. Aggression is something that every aquarium will have unless you keep a solo fish. Bioload is a term used to describe the amount of waste a fish contributes to its environment, or the total amount of waste an aquarium accumulates as a part of the Nitrogen Cycle. (What is the Nitrogen Cycle and how do I do it to my tank?) Bioload is also something every aquarium will have.
The first aspect to address is the potential size of your fish and the size of the tank it resides in. Obviously bigger is always better, but not everyone has the means to provide extra large aquaria for their fish. Not a problem as long as you know what is needed as a minimum. A good general rule of thumb for a minimum sized tank for any fish is this - L= 4x TL. (L = Lenth of the aquarium, TL = Total length of your fish) At a minimum you want the length of the aquarium to be four times longer than the length of your fish. Keep in mind this is a minimum and though it may suit some of your needs better, it doesn't always mean it will be easier. Keeping any given fish in the smallest recommended tank size will mean that you have to do water changes more often than providing a larger tank. The bioload doesn't change, but the amount of water is increased therefore reducing the Nitrate creep. A fish guru once told me “Minimum size tank equals maximum maintenance”.
The other aspect of size you will need to address is the size of the fish you intend to keep together. It is not wise to mix a fish that will potentially reach 12” with one that potentially will reach 3”. Cichlids are all opportunists and given the chance they will eat smaller fish, other cichlids included. Sizes should be relatively similar or hiding places should be provided for smaller, less aggressive cichlids.
Oscar's Establishing Dominance
If you have ever been to a pet store that sells fish and wandered down the cichlid isle chances are you've seen a label next to the species name. 'Aggressive'. 'Semi-Aggressive'. 'Community'. These labels are all a generalization. They tell you that for the most part this type of fish is aggressive, or does well in a community setting ect, etc. However as these are a generalization a fish in the Aggressive category will not always be aggressive, and a fish in the Community category will not always be suitable for communities. This scale is also very broad. Two 'Aggressive' fish will not always match up well together. For example the Convict cichlid (Amatitlani nigrofasciatum) and the Firemouth cichlid (Thorichthys meeki) are both in the Aggressive category. Based upon my personal experience 4/5 times the convict is going to be more aggressive, more assertive, and more territorial. It is also true that keeping a fish in a smaller aquarium will increase its aggressive tendencies. Why? Cichlids in general need a territory or a space to call their own. The more space provided, the less pressure there is to find a suitable territory. As the available space decreases, the pressure to find and claim a territory increases thus increasing potential aggression and stress. Keep this in mind when you want to mix cichlids as it will highly dictate what size tank is needed. The more fish you have the more space you'll need to provide.
Keeping more than one cichlid in the same tank is always going to form a hierarchy. A dominant fish and a submissive fish, Alpha and Omega, a leader and a follower. Certain fish do better in certain places in the hierarchy.
The Top: Most any fish will do just fine at the top of the hierarchy. They are dominant, freely roam unchallenged, and have their pick of the territories. This can also equate to getting the most food and having the least amount of stress imposed. Some Alpha fish are going to be more aggressive than others and this is where numbers come into play. If you have just two fish in a tank the submissive will always bare the load of aggression and will be constantly stressed. If you have three fish, two who are submissive then the stress is split between the two. Having four fish, three submissive, etc., etc. This however, does not mean that submissive fish will not show aggression or fight amongst each other.
The Bottom: The Convict cichlid fares rather well at the bottom of the hierarchy. They are bold, aggressive, and very resilient. I've seen a 4” convict back down fish 2-3 times it's size. What is needed most in the Omega fish is a high tolerance of stress. They should be able to keep calm when chased, and be smart about how they approach larger dominant fish.
If you have a large group of fish (6 or more) it can be difficult to tell who is where in the heirarchy. You will probably be able to pick out the dominant male, and probably even the next dominant male in line, but after that it might be hard to tell which fish are next in line. Having more cichlids certainly disperses aggression. The more fish you have, the less chance there is of one fish being singled out and bullied. However, most Central/South American cichlids grow quite large. It's fairly common for cichlids to grow upwards of 12”, and providing for several big fish might not be an option for you. I've found through experience that the Middle and the Bottom of the heirarchy benefit from larger numbers, but be wary of the potential size of the fish you are keeping. Keeping too many fish in a small aquarium might reduce aggression, but it will increase stress dramatically, and the Bioload will also increase. The trick to this is finding a balance that keeps aggression at a minimum, but also allows for enough space for all fish to be satisfied.
One of the most important subjects you need to be familiar with when attempting to mix aggressive fish is how to reduce aggression. Even in peaceful community settings this knowledge can be useful. Reducing aggression can be addressed in a number of ways.
Territorial aggression can be handled if you know what it is the fish are trying to claim. I have seen cichlids fight over a rock, driftwood, caves, pots, plants, even aquarium equipment. Some fish even like to claim areas rather than specific items in the tank. Be sure you know what exactly it is the fish are fighting over so you can act accordingly. Next, decide what course of action will best resolve the aggression. If you have one cave in the aquarium and they seem to be fighting over it you may have to add more caves. If adding territory does not work, you may have to remove the aggressor temporarily. Leave the aggressor in a different tank for several days so the others can become comfortable. Re-arrange the decorations and re-introduce the aggressor. Another notable technique is breaking up their line of sight by using large rocks, cave formations, and tall plants. Placing the plants directly inbetween separate territories can be effective. Lastly, an aquarium divider can be used to separate two fish. Dividers are often used to breed aggressive species to prevent the male from killing the female (or vice versa) during courtship. Although this tool is useful it is somewhat unsightly and if it is needed for two fish to co-exist they should probably be kept in separate aquariums.
If you've been in the hobby long enough, chances are you've run into a Rogue. A Rogue is a term I like to use to describe a cichlid or any aquarium fish that is excessively aggressive. It maybe be a Red Devil, or a Texas cichlid, or it might be an Angel fish or a Guppy. Rogues do not follow the typical description of the species. They are notable in that they are relentlessly aggressive and they have been known to kill their tank mates. Rogues can be dealt with in a few ways. Most often, Rogues are kept solo. They are too aggressive for any tank mates except maybe a plecostomus or other type of catfish that is not viewed as a threat. Others use Dither fish to reduce aggression. Dither fish are most often schooling fish that can be used to distract an aggressive fish, or comfort shy skittish fish. Common dither fish are: Silver dollars (Metynnis argenteus), and Giant Danios (Devario aequipinnatus). Another technique used to reduce aggression is reducing the water temperature. High water temperatures increase metabolism in cold blooded animals and it is thought that reducing the water temerature also will reduce metabolism therefore reducing aggressive tendancies.
Breeding cichlids are unpredictable. We know that any breeding pair of cichlids will vigorously defend their territory/fry. Their hormones become increased and aggression is inevitable. I would avoid this situation at all costs. No one is comfortable under these conditions, not even the pair. Killing potential predators (any tank mate) is not uncommon. If there are any signs of courting (gill flaring, body swaying, guarding a specific area) between a male and a female I would remove one of the two immediately. Unfortunately it is the only way to ensure safety. If you want to breed cichlids, I recommend a tank specifically for the male and female and their fry. Also keep in mind that cichlids do not have to be the same species to breed. Certain cichlids of separate species can successfully breed. Even if they cannot successfully breed, if they are courting each other chances are their aggressive tendencies will increase. Avoid pairs of fish at all costs.
Be selective when choosing your fish. Make sure that the range of aggression is appropriate for the mix you are choosing. You want fish that are similar in aggression. Amphilophus labiatus (Red Devil) and Archocentrus sajica (T-bar Cichlid) are not a good mix. Red devils tend to be very aggressive and reach 12-14”. Sajica's are peaceful and only reach about 5”. Also remember that many fish have higher conspecific aggression than others. This means that they will tend to be more aggressive toward fish that resemble their own species. Choosing all of your fish from the Amphilophus or Parachromis genus may not be a good idea. Size difference can work to your advantage as much as it can to your disadvantage. Many large cichlids are only aggressive toward fish of similar size as they are the only ones that pose a threat to their territory. Smaller fish are ignored often when the size difference is vast. Though, be wary, as the size difference increases so may the menu.
I'm convinced that the perfect aggressive cichlid community is rare. Even when you've put in years of dedication to an aggressive community all it takes is one small mistake for all hell to break loose. That's just the risk of dealing with aggressive cichlids. They are unpredictable at times but that is also part of what makes cichlids interesting. You may have an aggressive community that is going great but unless all of your fish are full grown it is hard to predict future success. This is a long term committment. It is easy to forget that these fish can live upwards of 10 years in good care. Aggression should be watched very closely. When a fish becomes physically weakened due to stress, injury, or poor conditions something needs to be done. Torn fins and scrapes are going to happen. It's pretty much unavoidable. You have to be the judge and say 'enough'. Remove fish that are sick, injured, or too aggressive, and quarantine new fish. Hobbyists are all too familiar with moving tanks, selling fish, buying fish, and breeding fish. Success here is learning from your mistakes and working toward a perfect community. Education is part of the hobby and in my opinion, the most important.
The majority of the knowledge you need to be able to create a successful aggressive community aquarium will come with experience. Be patient, be attentive, and you will be happy with the results.