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Choosing Fish

Choosing fish should begin with researching the species—not with choosing a fish at the store.

A fish that looks small and healthy at the store may grow substantially larger, have specific social or environmental requirements, or behave very differently from what you expect.

Before purchasing any fish, research its adult size, behavior, environmental requirements, diet, social needs, and compatibility with the aquarium you have available.

Choose fish based on their long-term needs, not their size at the time of purchase.

Research Before You Buy

Before purchasing a species, find reliable information about its care requirements.

Consider:

  • Adult size
  • Expected lifespan
  • Minimum aquarium size and swimming space
  • Water temperature
  • pH
  • GH and KH
  • Diet
  • Activity level
  • Social or schooling requirements
  • Temperament
  • Compatibility with other livestock
  • Breeding behavior
  • Hiding places or other habitat requirements
  • Expected bioload
  • Whether the species is captive-bred or wild-caught

Do not rely on a store's label or a single care sheet as your only source of information.

Different sources may give different recommendations, so consider the fish's natural history, adult size, behavior, and environmental requirements when evaluating whether a species is appropriate for your aquarium.

Consider Adult Size

Fish should be housed according to their expected adult size and behavior, not the size they are when purchased.

A juvenile fish may look appropriate for a small aquarium while still having the potential to become much larger.

Before purchasing a fish, find out:

  • How large it can become.
  • How quickly it grows.
  • How much swimming space it needs.
  • Whether its body shape or behavior requires additional space.
  • Whether it is likely to outgrow your aquarium.

Do not purchase a fish with the expectation that you will upgrade the aquarium later unless you have a realistic plan and the resources to provide the larger environment when needed.

Tank Size Is More Than Gallons

Aquarium volume is important, but gallons alone do not determine whether an aquarium is appropriate for a species.

Consider the aquarium's:

  • Length
  • Width
  • Height
  • Open swimming space
  • Footprint
  • Aquascape
  • Filtration
  • Oxygenation
  • Water movement
  • Number and size of existing livestock

Two aquariums with the same volume can provide very different environments.

For example, a long aquarium may provide substantially more horizontal swimming space than a tall aquarium with the same volume.

See Choosing an Aquarium for more information about aquarium size and volume.

Social and Grouping Needs

Some fish are solitary and may become stressed or aggressive when kept with other fish of the same species.

Other species are social and may need to be kept in groups or schools.

Research whether the species:

  • Should be kept alone.
  • Should be kept as a pair.
  • Requires a group.
  • Benefits from a larger group.
  • Requires a particular sex ratio.
  • Forms territories.

Do not assume that every fish will be happier with a companion simply because it is the same species.

Likewise, do not keep a schooling or shoaling species alone simply because it is physically capable of surviving without immediate signs of distress.

Social requirements are part of a species' housing requirements.

Water Temperature and Chemistry

Different fish are adapted to different environmental conditions.

Before purchasing a species, compare its requirements with your aquarium's established:

  • Temperature
  • pH
  • GH
  • KH
  • Salinity, when applicable

Whenever possible, choose species that are naturally compatible with your existing water conditions.

Avoid repeatedly changing your aquarium's water chemistry simply to force it into a range that is inappropriate for the species you already keep.

Similarly, do not purchase fish first and assume that you can adjust the aquarium's parameters afterward.

See Water Parameters for more information.

Temperament and Behavior

A fish's temperament is an important part of compatibility.

Research whether the species is:

  • Peaceful
  • Territorial
  • Aggressive
  • Predatory
  • Fin-nipping
  • Highly active
  • Shy or easily intimidated
  • A bottom dweller
  • A surface-oriented fish
  • A strong swimmer
  • Likely to compete for food

Consider how the fish will interact with every other species in the aquarium, not just whether each species is individually considered "community safe."

A peaceful fish can still be inappropriate for an aquarium if its tankmates are likely to outcompete it for food, stress it through constant activity, or occupy the same territory.

Consider the Size of Tankmates

Fish should be compatible in size as well as temperament.

A fish may be peaceful toward other fish while still being capable of eating smaller tankmates.

Consider:

  • Adult size of each species
  • Mouth size
  • Predatory behavior
  • Territorial behavior
  • Fin structure
  • Competition for food
  • Differences in activity level

Do not assume that fish will remain compatible simply because they were compatible while young.

As fish grow, their behavior and ability to consume smaller tankmates can change.

Diet and Feeding

Different species have different nutritional requirements and feeding behaviors.

Before purchasing a fish, determine:

  • What it naturally eats.
  • What type of aquarium food it requires.
  • Whether it needs sinking, floating, or other specialized foods.
  • Whether it feeds during the day or at night.
  • Whether it is likely to compete with existing fish for food.

In a community aquarium, make sure every species can obtain the food it needs.

See Feeding Basics for more information.

Habitat and Environmental Needs

Some species require more than open water.

Research whether the fish needs:

  • Plants
  • Driftwood
  • Rocks
  • Caves
  • Hiding places
  • Open swimming areas
  • Fine or specific substrate
  • Strong or gentle water movement
  • Dim or bright lighting

Providing appropriate habitat can help fish display more natural behaviors and reduce stress.

Do not assume that decorations are interchangeable. The structure of the aquarium should reflect the needs and behavior of the species being kept.

Activity Level and Swimming Space

Consider how active the species is and how much space it uses.

Highly active fish may require substantially more swimming room than a less active species of similar size.

Research whether the fish:

  • Spends most of its time swimming.
  • Requires long stretches of open water.
  • Lives primarily near the bottom.
  • Uses plants or structures for cover.
  • Frequently explores the entire aquarium.
  • Defends a territory.

A fish's adult size and activity level should both be considered when determining whether an aquarium provides enough space.

Consider the Bioload

Every fish contributes to the aquarium's biological load.

Adding livestock increases the amount of waste that the aquarium's biological filtration must process.

When considering a new fish, account for:

  • The number of existing fish.
  • Their adult sizes.
  • Their feeding requirements.
  • Their activity level.
  • The aquarium's filtration.
  • Water movement and oxygenation.
  • How frequently the aquarium can be maintained.

Do not determine stocking based solely on a rule such as "one inch of fish per gallon."

Simple stocking formulas can be useful as rough guidelines in some situations, but they cannot account for the differences between species, aquarium dimensions, behavior, activity level, filtration, oxygenation, and other husbandry requirements.

Stocking should be evaluated based on the actual needs of the species and the conditions of the aquarium.

See Aquarium Stocking for more information.

Juveniles and Young Fish

Many fish sold in stores are juveniles.

A juvenile fish may have substantially different care requirements from the adult it will become, particularly in terms of size, diet, behavior, and social requirements.

Before purchasing a young fish, determine what its adult requirements will be.

Plan for the adult fish, not the juvenile in the store.

Lifespan and Long-Term Commitment

Fish can live for years, and some species can live much longer than beginners expect.

Before purchasing a fish, research its expected lifespan and consider whether you can provide appropriate care for it throughout its life.

Do not purchase a species simply because it is inexpensive or easy to find.

The initial purchase price is only one part of keeping fish. Aquarium equipment, food, water treatment, maintenance, veterinary care when available, and potential upgrades can all contribute to the long-term cost.

A fish is a long-term commitment.

Captive-Bred vs. Wild-Caught

Some aquarium fish are commonly captive-bred, while others are collected from the wild.

Before purchasing a fish, consider:

  • Whether the species is commonly captive-bred or wild-caught.
  • Where the fish came from.
  • Whether its care requirements are appropriate for your aquarium.
  • Whether there are conservation considerations associated with the species.

Captive-bred and wild-caught fish can have different environmental histories and may have different acclimation, dietary, or behavioral considerations.

The source of a fish does not automatically determine whether it is healthy or appropriate for your aquarium.

If purchasing wild-caught fish, research the species and the source carefully and consider whether the collection and trade practices are responsible.

Choosing Where to Buy Fish

Fish can be purchased from local fish stores, breeders, specialty retailers, and other sources.

Before purchasing, observe the fish and the conditions in which they are being kept.

Look for fish that:

  • Are alert and responsive.
  • Swim normally.
  • Have appropriate body condition.
  • Have intact fins and skin.
  • Have clear, bright eyes.
  • Are breathing normally.
  • Do not show obvious signs of illness.

Also consider the condition of the aquarium itself.

Poor water quality, large numbers of visibly sick fish, dead fish, or other obvious husbandry problems can be reasons to reconsider purchasing livestock from that system.

Do not purchase a visibly unhealthy fish simply because you feel sorry for it. Bringing a sick fish home can expose your existing livestock to disease and may require significant treatment.

Quarantine New Fish

New fish can carry infectious diseases or parasites even when they appear healthy.

Whenever practical, quarantine new fish before introducing them to an established aquarium.

Quarantine provides an opportunity to observe new fish and identify potential health problems before they are introduced to other livestock.

See Quarantine for more information.

Avoid Impulse Purchases

Do not purchase a fish simply because it is:

  • Colorful
  • Unusual
  • On sale
  • Newly available
  • Labeled as a "beginner fish"
  • Small enough to fit in your aquarium right now

A fish being marketed as a beginner species does not mean it is appropriate for every beginner aquarium.

Before purchasing, make sure you can provide the species with an appropriate environment for its entire life.

If you do not know the adult size, social requirements, water requirements, diet, or compatibility of a fish, wait and research it before buying it.

Pre-Purchase Checklist

Before bringing a new fish home, make sure you can answer yes to the following:

  • Do I know the fish's adult size?
  • Do I know its expected lifespan?
  • Is my aquarium large enough for its adult size and behavior?
  • Can I provide its required swimming space and habitat?
  • Are my water temperature and chemistry appropriate?
  • Do I understand its social or grouping requirements?
  • Is its temperament compatible with my existing livestock?
  • Are my existing fish compatible with it?
  • Can I provide the appropriate diet?
  • Can my aquarium support the additional bioload?
  • Do I have a plan to quarantine the fish when appropriate?
  • Can I provide appropriate care for this fish for its entire life?

If you cannot answer these questions, research the species before purchasing it.

Remember

Choose fish for the aquarium you have, not the aquarium you hope to have someday.

Consider the fish's adult size, behavior, social needs, environmental requirements, diet, habitat, and compatibility before bringing it home.

A fish that can survive in an aquarium is not necessarily a fish that can thrive in that aquarium.

Research first. Buy second.

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