What is a Community Fish Tank?


PetMD Editorial

Updated Aug. 16, 2022

By Kenneth Wingerter

It is patently human to collect things. Some people collect shoes, some collect postage stamps, and some of us collect fish.

When collecting fishes, or any living things, we are burdened with the task of ensuring that our “specimens” are provided with the best possible living conditions. This becomes increasingly difficult as our collection becomes larger and more diverse. In some cases, this may mean that a number of species tanks are maintained. In most cases, however, aquarium hobbyists opt to keep a variety of fishes in a single community tank.

How Many Fish Can Live in One Tank?

Community fish tanks present special challenges. Of course, one must determine the largest number of fishes that their tank can adequately house. Aquarists are often advised to stick to one inch of fish per gallon of aquarium water volume. While this rule may suffice as a very basic guideline, it should be regarded as arbitrary. There are simply too many other factors at play here.

Particulars such as feeding/maintenance regimen and type/size of filter can significantly influence the maximum manageable density of animals (sometimes referred to as bioload). Stocking in excess of a system’s bioload capacity can result in a dirty, unsightly display at best, or, at worst, poor water quality and fish death.  

Another reason that the inches/gallon metric is imperfect is that it fails to take into account (1) the potential for competition/predation, and (2) different fish species’ different use of living space.

For example, it is probably much more feasible to successfully keep 30 inches of neon tetras (which are peaceful schoolers) than 10 inches of mbuna African cichlids (which are highly aggressive and territorial) in a 20-gallon tank. Thus, when setting up a community tank, one must carefully assess the compatibility of all prospective species to be added. That is, the selected species must not only be capable of thriving in the same environment (with respect to temperature, pH, water flow rates, salinity, etc.), but they also must be able to play well with each other. 

Which Fish Can Live Peacefully Together?

Species surely is a major indicator of whether or not two fishes will play well together. Some aquarium-keeping books even include fish compatibility charts as a handy guide. But real life compatibility between animals can hardly be determined by species alone.

There are many factors that influence the temperament and space needs of an individual fish. These include size, age, sex, and life history (i.e., its social environment prior to introduction in your aquarium). The tank’s physical environment can likewise affect interactions between individual fishes. These might include aquascape characteristics or even light intensity. Because there are so many things that can affect a captive fish’s behavior, an aquarist must take a well-informed course of action (and take a few educated guesses) while stocking a community fish tank.

As the community grows and becomes more varied, it will become harder and harder to find new, companionable tankmates. Here, compatibility goes beyond the fishes simply getting along with each other. Your choices will be determined by potential predator and prey relationships, and by feeding behaviors. For example, if you want to include ornamental shrimp or live plants in your aquarium, you will need to choose fish that do not prey on invertebrates or graze on live aquarium plants.

Creating Safe Spaces for Your Fish

As simple and primitive as they might appear, individual fish can possess surprisingly unique personalities. While some of these personality traits arise from genetics, many are simply the result of learned behaviors. As such, fish can be trained or manipulated into behaving in a favorable manner. Indeed, there are a few precautions one can take to help ensure that a community of fish will harmoniously coexist.

The first such action involves the creation of an ideal aquascape. For one, it is always a good idea to provide ample swimming space for active species. These relatively open areas should be made over the bottom (for horizontal, or back and forth, swimming) as well as in the open water column (for vertical, or up and down, swimming). Also, and perhaps most importantly, there should be an abundance of hiding places. A nice variety of hides can be created with sunken branches, rocky ledges, and plant thickets. In addition to making discreet hideaways, erecting partitions with larger plants, stones, etc. will help to break up the terrain and thus minimize territorial disputes. 

When planning out the fish community, one should be sure to choose species that inhabit different parts of the tank. For example, one might select a Pantadon butterflyfish to reside at the surface, a pair of discus fish to reside in the midwaters, and a troop of clown loaches to reside on the bottom. By merely staying out of each other’s way, tankmates will be less likely to squabble over space or food.

Newcomers are by far the most likely to experience bullying. Thus, order of introduction can greatly affect the outcome of fish interactions. To give smaller, slower, or more timid species a valuable edge, it may be best to add the more aggressive or predatory species last.

The Reward for Careful Planning

Bearing these considerations in mind, we might regard a community fish tank as something more than a mere hodgepodge of randomly picked species. Any mixed tank might hold species based on fairly superficial criteria (such as color), whereas a true community tank will house a carefully selected population of ecologically complimentary species.

To be sure, community aquaria can be restrictive with respect to the number of individuals and species that can be kept in one tank. These small limitations do, however, have a big payoff: happier and healthier fish!

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