Once you have furnished your aquarium with the correct equipment and cycled the tank, you can start the most exciting step of setting up a new tank: stocking the community. This may include the fish you have already used to cycle the tank, and any other fish, invertebrate, and plant species you would like to keep.

You may have a certain species in mind and design a tank and community to suit it; for example, if you would like to keep neon tetras, choose other species that would complement them and also enjoy a densely planted, aggression-free tank. If you have a certain ecosystem in mind, such as a Caribbean coral reef, you might choose only species that would naturally be found in this environment.

While it is true that species of a common geographic origin are more likely to share the same environmental preferences, it is certainly possible to keep fish from all over the world in the same tank – as long as they are suited temperamentally and have similar preferences for temperature, water quality and chemistry.

Number of fish

In freshwater tanks, the general rule of thumb is one half to one inch of fish per gallon of water. Gravel and decorations do not count towards your gallons of water – a ten gallon tank likely only holds 8 or 9 gallons of water once set up. In marine tanks, the rule is closer to one inch of fish per 5 gallons.

The full-grown size of the fish must be considered, rather than the purchase size, unless you intend to upgrade your tank later on. A one-inch fish that will eventually grow to six inches must be allotted at least 6 gallons of water. Keep in mind, also, that a one-inch fish that lives quite peacefully with other one-inch fish may see its tank-mates in a new, tasty light once it has gained five inches on them.

Environmental preferences

To a certain extent, there is room even in a small aquarium for some variation in the environment; for example, one end of the tank may be shady and densely planted, while the other is open and well-lit. However, some factors such as temperature and acidity will be fairly constant throughout the tank, and should be well suited to all the species you intend to keep. For this reason, many aquarists choose a specific ecosystem – such as a South American community, with tetras, catfish, and swordplants.

The more common species of aquarium fish (such as guppies, goldfish, and tetras) are usually captive bred and more adaptable than their wild-caught counterparts. They are often highly compatible with an ‘average’ aquarium temperature and chemistry and therefore well-suited to a mixed-species community (depending on temperament).

Behavioural considerations

The individual role or niche that each species will play in the tank community should be considered. Most aquariums have at least one species to act as a scavenger or bottom-feeder, such as a few loaches or snails. To provide visual variety and to prevent competition among tank-mates, you might also choose a variety of middle-feeding, schooling species, and surface-feeding labyrinth species. Some tanks even have separate diurnal (daylight-active) and nocturnal (night-active) groups to ensure activity at all times.

Although it is not true that predatory fish cannot be kept in a community environment, their temperament and feeding preferences must certainly be considered when selecting tank-mates. Some herbivorous species can also be quite aggressive. Hyper or aggressive species can harass and injure passive or slower-moving fish even if they do not intend to eat them. Larger tanks may successfully house both types of fish if they provide enough visual barriers (such as plants and decorations) and hiding places for the more timid fish.

Shoaling species, such as danios, prefer to be kept in small groups, while others, such as pufferfish, are highly territorial and likely to be very aggressive if kept with members of the same species.

Introducing fish

Once you have decided on the number and type of species (fish, plants, and invertebrates) you would like to keep, make a list and stick to it. Occasionally you may find that a species you were hoping to keep is not locally available, so be aware of other species that might make good substitutions.

Build up the stock in your tank over the course of a few weeks, in order to allow time between introductions for both the aquarium and the residents to adjust to the new additions. You might consider holding new purchases in a quarantine tank for some time before introducing them, to avoid contaminating your community.

Avoid the temptation to impulse buy at the store, and take the time to do a little research on the species, first – a well-planned community is a happy one.

By Laura Platt – Pets.ca writer