Choosing fish that get along is a challenge every aquarist faces. While there are certain combinations we know with relative certainty do or do not work in most instances, there are countless others that can go either way depending on a variety of factors.
Here are some things to keep in mind when purchasing fish for your aquarium that can help determine fish compatibility.
Most fish need space, and the more they have the better they tend to get along. When fish are crowded they become more agitated and are more likely to quarrel with tank mates. A general rule of thumb for stocking a fish tank is one inch of adult size fish per net gallon of aquarium capacity, but territorial fish need even more space. Remember that the fish you buy will probably grow, and a 30 gallon aquarium doesn't actually hold 30 gallons of water when you factor in internal dimensions, gravel and decorations. Also, what to us is a large aquarium (200+ gallons), is still just a fraction of the space fish have in their natural habitats.
Another consideration are the dimensions of your new aquarium. Different fish prefer different shapes and swimming spaces. Wider aquariums give active fish, like danios and barbs, the space to spread out, which in turn helps them get along better. On the other hand, tall, narrow aquariums are appealing to look at and fit into narrow spaces but don't offer fish as much swimming space or territory as a wider aquarium. These aquariums should be used for less active fish like discus, angelfish and gouramis.
Decorations and Plants
Aquarium decorations help with saltwater and freshwater fish compatibility in several ways. Most fish need a place to call their own and they define their personal areas by physical boundaries. In addition, when they can't see each other, they tend to mind their own business. Rocks, caves, driftwood and other decorations help define territories for cichlids and other territorial fish, while tall bushy plants provide habitat and give schooling fish like tetras, barbs, danios and rasboras their own areas to occupy. When introducing new cichlids to existing populations, add a few new rocks or other decorations and rearrange existing décor to eliminate territories controlled by established fish.
Species/Origin of the Fish
Fish communicate in a variety of ways, and signals can be misinterpreted because fish from different parts of the world "speak different languages". Research fish before buying and try to stock your aquarium with fish from the same region, especially if they are aggressive or territorial species.
Cichlids, certain species of sharks, loaches, knife fish, mormyrids and other territorial fish do not share space well with members of their own kind or closely related species. Large aquariums with plenty of cover help, but many of these fish are best kept individually and tank mates should not be similar looking or closely related.
Asking your local retailer about how to build a community fish tank, featuring a variety of species is always a good option. Finding an appropriate fish compatible with bettas, for example, can be challenging but your local expert can help advise you as needed.
Juvenile fish are usually easy going, even if they are known to become aggressive as adults. They can often be mixed with a wider selection of tank mates, which they'll accept as they grow and mature. There are many reports of Oscars and other large predatory fish coexisting with feeder goldfish that they could easily swallow but don't because both fish were purchased when they were the same size, before the predator grew larger and learned to eat other fish.
An old aquarium adage states "if a fish can fit into another fish's mouth, chances are it will end up there." Most fish are opportunistic when it comes to food, and even relatively peaceful fish will try to eat other fish if they think they can. Always purchase fish that are roughly the same size as those in your aquarium. When mixing territorial fish, newcomers should be at least the same size as the largest or most aggressive fish already in the tank.
Male fish tend to be more territorial and aggressive, particularly when mating. This is especially true for cichlids. Avoid having more than one male of the same or closely related cichlid species or other territorial species, especially if females are present. Keep livebearers in ratios of 2 to 3 females per male to diffuse the persistent mating behavior of males.
Quantity of Aquarium Fish
Boisterous fish like tiger barbs and even certain species of tetras and danios are often better behaved when in large schools. Shy schooling fish are less likely to be picked on when kept in larger numbers as well. Always buy schooling fish in groups of 6 or more. While most fish get along better when they have more space, African Rift Lake Cichlids are an exception; they do best when crowded a little as this deters dominant fish from singling out weaker or submissive fish.
These are especially common in cichlid communities, where distinct pecking orders exist. Smaller or submissive fish that are harassed may need to be removed. Removing the aggressor is another option, however, the next fish on the totem pole may quickly assume the dominant role, continuing the cycle. When adding new fish, make sure the aquarium is large enough and has enough cover to accommodate newcomers, and purchase fish that are at least as large as the biggest or most aggressive fish in your aquarium.
We often think of predatory fish as large, aggressive creatures, typically in the Family Cichlidae. But there are other types of predators, such as catfish, halfbeaks, leaf fish, needlefish, bichirs, certain gobies, arowanas, stingrays and gar, to name just a few. For these fish, whatever they think they can fit in their mouth is fair game. It doesn't make them mean or aggressive, it just means they're hungry. Many catfish species are nocturnal and come out at night to hunt small fish resting on the bottom.
If Fish Are Breeding
All fish that practice parental care – mostly, but not restricted to, cichlids – become especially defensive at breeding time. They will take over and control large areas of the aquarium, pushing all other inhabitants into a far corner. Anyone who has had a normally peaceful pair of angelfish spawn in their aquarium can attest to this. Be prepared to move "prospective parents" to a dedicated breeding tank.
While we can generalize about the typical behavior patterns of a given species, there are always exceptions to the rule. All of the factors mentioned above can affect how fish interact in an aquarium, but at the end of the day, fish have individual personalities and sometimes they behave in uncharacteristic ways. The best approach is to expect the unexpected and be prepared to separate fish that do not get along.
Always do plenty of research before buying new fish for your aquarium and leave yourself options for relocating fish that don't work out, especially in those "iffy" situations.