Fish Compatibility and Community Tanks
How to match aquarium fish by temperament, adult size and water parameters, and which species and traits make tanks incompatible.
Overview
A community aquarium brings together species that are compatible in temperament and water requirements. Compatibility is judged on several axes at once: how peaceful or aggressive a fish is, how large it grows, what water chemistry it needs, and where in the tank it lives. When these align, a stable and peaceful community is possible. No single trait decides the matter, so a fish that is peaceful but far too large, or perfectly sized but aggressive, is still a poor community choice.
Match water parameters
Tankmates must share a tolerable range of temperature, pH and hardness. Most freshwater community fish do well in water that is soft to moderately hard with a pH between 6 and 8. Mixing species with opposite requirements, such as soft-water blackwater fish with hard-water livebearers, forces a compromise that suits neither group. Choosing species whose preferred ranges overlap means the tank can be kept at one set of parameters without leaving any resident chronically stressed.
Match adult size
Size compatibility is critical because predation in aquaria is opportunistic. If one fish fits into the mouth of another, there is a real chance it will be eaten by the larger one. Pairing similar-sized species is the simplest way to avoid losses, so the adult size of every species, not its size at purchase, should guide the selection.
Match temperament
Peaceful community species include many livebearers, barbs, tetras, rasboras, danios, rainbowfishes, gouramis and Corydoras catfishes. Territorial and predatory fish, such as many cichlids, are poorly suited to mixed communities. Nervous or specialised feeders may also struggle to compete for food with bolder, faster tankmates.
Watch for fin-nippers
Some otherwise community-suitable fish nip the fins of slower or long-finned tankmates. Tiger barbs and serpae tetras are commonly cited fin-nippers. Their nipping is often worse when they are kept in too-small groups or with insufficient space, so adequate numbers and swimming room reduce the problem.
Species to avoid in communities
- Predatory fish such as snakeheads, leaffishes and bucktooth tetras.
- Territorial fish such as red-tailed black sharks that defend an area.
- Many cichlids that become aggressive when breeding or guarding territory.
- Fragile or nervous species that are easily outcompeted or stressed.