Fish are some of the most diverse animals on the planet, with some 15,000 species in freshwater alone. So it probably comes as no surprise that not all of them get along or can be housed in the same aquarium in captivity. Whether it’s potential predation, aggression, differing environmental needs, or competition for food or different dietary needs, not all fish can cohabitate successfully. To determine which fish can live with which, one has to look at a few factors.

Eventual Size

No matter how peaceful two species of fish are, if one can fit in the other’s mouth, there’s a good chance that’s where it will end up. So it’s important to research how big your fish will get before mixing and matching nano fish with tank busters. Just because they’re the same size when you buy them doesn’t mean they’ll stay that way.

Aggression Level

Some fish are more territorial than others by nature, and therefore can relentlessly chase around tankmates or beat up conspecifics. Male bettas, for instance, will not tolerate other males and will fight to the death if put in the same aquarium. Some fish are nippy and will shred fins on more ornate or slower species. It’s best, when possible, to mix fish of similar temperament and stock accordingly.

Environmental Needs

Some fish cannot go together simply because they have conflicting requirements. Discus, for instance, cannot cohabitate with coldwater fish such as goldfish. This is because discus come from the tropical Amazon River and need temperatures in the 80’s to thrive. Some fish need blackwater (acidic) environments and some need alkaline (basic) environments. Some fish need high flow replicated by powerheads and some need calmer waters. If fish come from similar environments in the wild, there’s a good chance they’re compatible in captivity.

Feeding Requirement

Stocking boisterous fishes with more shy fish can result in the shy fish not eating. Small, diminutive or specialized feeders are often best kept in species only tanks to avoid being outcompeted for food. These are animals such as Scarlet badis (Dario), micro rasboras (boraras, sundadanio), mormyrids (elephantnose), and other oddballs such as pipefish and halfbeaks. There are also predatory and piscivorous species to watch out for (Gar, bichirs, belonesox), whose feeding requirements might mean their tankmates are on the menu, regardless of size. Other fish have such specialized dietary needs that eating the wrong food that a tankmate might enjoy could be detrimental. Luckily, this isn’t most fish in the trade, but it is something to be aware of (Tropheus, for instance, require an algae based diet).

Conclusion

By mixing fish of similar size, from similar environments, and with complimentary aggression levels and feeding habits, you can have a very successful aquarium. The idea is to have fish as comfortable and stress free as possible. Stress, after all, can quickly lead to disease and death in any organism.  Whether you’re recreating a biotope from the wild or making your own community, following the above considerations will help you succeed and avoid compatibility issues in your aquarium.