With over 33,000 known species of fish in the world, it can be tricky to identify all the small fish out there. If you’re wondering ‘what is a small fish called?’, you’ve come to the right place.

If you’re short on time, here’s a quick answer to your question: Small fish are generally classified based on their adult size, with fish under 4-6 inches considered small fish. Some common examples of small fish are minnows, killifish, gobies and blennies.

In this comprehensive guide, we’ll provide an in-depth look at small fish species. We’ll cover what defines a small fish, give examples of popular small fish kept in home aquariums, outline small fish species in the wild, and go over how to properly identify small fish.

Defining Small Fish

Size Classification

When it comes to defining what constitutes a small fish, size is the most obvious factor. Generally, small fish are considered to be those that reach adulthood at a maximum length of around 4-8 inches (10-20 cm). However, there is no universally agreed upon size cutoff and classifications can vary.

Here are some general guidelines for categorizing fish size:

  • Extra small fish: Less than 2 inches (5 cm) maximum adult length
  • Small fish: 2-4 inches (5-10 cm) maximum adult length
  • Medium fish: 4-12 inches (10-30 cm) maximum adult length
  • Large fish: Over 12 inches (30 cm) maximum adult length

So a “small” fish is usually one that does not surpass 4-8 inches in length once fully grown. This includes many species commonly kept in home aquariums, such as neon tetras, guppies, and betta fish. In the wild, small fish occupy important ecological niches and make up a significant proportion of lake, river, and ocean ecosystems.

Other Defining Traits

While size is the primary factor, there are other traits that can characterize small fish species:

  • Short lifespan – Many small fish only live for 1-3 years in the wild.
  • Rapid maturation – They often reach sexual maturity within their first year.
  • High reproductive rate – Small fish tend to produce hundreds or even thousands of eggs per spawning.
  • Schooling behavior – They often swim together in schools for protection from predators.
  • Generalist feeding – Omnivorous diets consisting of zooplankton, insects, plant matter.
  • Important prey species – Small fish are a vital food source for larger predatory fish, birds, and other wildlife.

Additionally, some small fish inhabit unique environments like tidepools (sculpins) or coral reefs (gobies, blennies). Others are notable for distinct survival adaptations like being able to live in waters with very low oxygen (killifish) or surviving drought conditions by burying themselves in mud (lungfish).

So while size is the clearest determinant, behavior, physiology, habitat, and ecology can also help characterize a fish species as “small.” Paying attention to these kinds of traits is important for understanding the diversity within this group and the role these abundant fish play in aquatic ecosystems.

Popular Small Fish for Home Aquariums

Freshwater Species

When it comes to freshwater aquarium fish, some of the most popular small species are tetras, rasboras, danios, dwarf cichlids, and dwarf catfish. Neon tetras with their bright colors are an absolute classic, while ember tetras and rummy nose tetras also make excellent choices.

For schools of tiny fish, galaxy rasboras, chili rasboras, and phoenix rasboras are great options. Lively zebra danios and white cloud mountain minnows are perfect for cold water aquariums. Among the dwarf cichlids, apistogramma and ram species thrive in planted tanks.

For the bottom level, cute little corydoras catfish and otocinclus eat algae.

When choosing small freshwater fish, it’s best to select peaceful community species that can live together without aggression. Tetras, rasboras, and danios commonly school together in the middle water column. Dwarf cichlids and catfish inhabit the lower levels.

Make sure any species you select share similar water parameters in terms of temperature, pH, and hardness for a thriving aquarium. Provide plenty of plants and hiding spots. Overcrowding causes stress, so follow general stocking guidelines of 1 inch of adult fish per gallon of water.

Saltwater Species

For small saltwater aquarium fish, damselfish, blennies, gobies, and clownfish are excellent starter choices. Neon gobies and clown gobies have tons of personality. Blennies like lawnmower blennies and tailspot blennies are full of quirks. Clownfish remain a popular staple.

While anemonefish like percula, ocellaris, and tomato clowns typically pair up. Chromis, especially the blue green reef chromis, school nicely. And you can’t go wrong with pajama cardinalfish or a flame hawkfish for a splash of color.

With saltwater fish, focus on species ideal for nano and pico reef tanks under 30 gallons. Avoid fast swimmers that need lots of room like surgeonfish and large angelfish. Select peaceful fish that won’t nip at corals or invertebrates. Herbivores like blennies help manage algae growth.

Mix species that occupy different tank zones from the substrate to mid-water. Don’t overstock the tank, allowing 3-4 small fish for a 15-20 gallon tank. Maintain excellent water quality with proper filtration, lighting, and stable water parameters.

Freshwater Fish Saltwater Fish
  • Neon tetras
  • Galaxy rasboras
  • White cloud minnows
  • Corydoras catfish
  • Clown gobies
  • Blennies
  • Clownfish
  • Cardinalfish

Researching species specifics, providing proper care and tank conditions, and sourcing healthy fish from reputable retailers will lead to success. For more detailed fish profiles and care guides, check out resources like Fishlore and LiveAquaria.

Small Fish Species in the Wild


Minnows are a diverse group of small freshwater fish that are members of the Cyprinidae family. There are over 2,000 species of minnows found throughout North America, Europe, and Asia. Some of the most common minnow species include the fathead minnow, bluntnose minnow, and creek chub.

Minnows typically range from 1 to 6 inches in length. They have slim, streamlined bodies that allow them to swim and maneuver easily in moving water. Most minnows have an olive green or silvery coloration which helps camouflage them from predators.

Minnows play an important role in freshwater ecosystems. They serve as a key food source for larger predatory fish such as bass, pike, and trout. Minnows are omnivores that feed on algae, plankton, insects, and plant matter.

Some species of minnows school together in large groups which offers protection from predators through safety in numbers.

While naturally occurring, minnows are also frequently used as bait fish by anglers looking to catch bigger game fish. They can easily be caught via seine nets and minnow traps. Overall, minnows make up a diverse and integral part of freshwater habitats across the Northern Hemisphere.


Killifish are a family of small, vibrantly colored fish found in freshwater and brackish waters primarily in the Americas and Africa. There are over 1,300 species of killifish known worldwide. Some popular species kept in home aquariums include the clown killifish, bluefin killifish, and Fundulopanchax Gardneri.

Most killifish grow to around 2 to 3 inches in length, with some species only reaching 1 inch full grown. They are appreciated for their bright color patterns which can include metallic blues, vibrant yellows, fiery reds, and iridescent greens.

Male killifish tend to be more vividly colored than females of the same species.

Killifish inhabit shallow, often seasonal waters including streams, rivers, ponds, and marshes. They have adapted to thrive in conditions that would kill other fish, like low oxygen levels, high temperature, high pH, and high salinity.

Some killifish even inhabit rain puddles, hoof prints, and ditches which frequently dry up for months at a time. When their habitat dries up, killifish eggs can remain dormant in dried mud for up to several years.


Gobies are a family of small, mostly bottom-dwelling fish comprising over 2,000 species found in marine waters, brackish estuaries, and freshwater streams across the world. Some of the most abundant goby species kept in home aquariums are neon gobies, dragon gobies, and shrimp gobies.

Gobies are characterized by their fused pelvic fins that form a suction disc. This allows them to cling to objects in fast moving water and grip surfaces like rocks, coral, and the sandy bottom. Most gobies grow to around 2 to 4 inches in length.

They have elongated, tapered bodies and large, highly adapted eyes.

Gobies come in a stunning array of colors like yellow, blue, orange, pink, and green. Their coloration helps gobies camouflage and hide from predators. Gobies feed on small invertebrates like worms, crustaceans, insect larvae.

Some unique goby species even form symbiotic relationships with blind pistol shrimp or live in the mouths or gills of large invertebrates for protection! Overall gobies demonstrate amazing diversity and adaptability, allowing them to thrive in aquatic habitats globally.


Blennies are a family of small, often colorful fish comprising over 800 species found along rocky shorelines and coral reefs worldwide. Some popular aquarium blenny species include lawnmower blennies, peacock blennies, and scooter blennies.

Most blennies reach around 3 to 6 inches long. They have elongated, slender bodies and unique head shapes characterized by a continuous dorsal fin and a lack of scales. Blennies come in a rainbow of colors like red, yellow, green, blue, orange, and brown.

Their coloration and ability to change color helps camouflage them against predators.

A key adaptation of blennies are their ventral fins that act like small limbs, allowing them to grip onto rocks, crevices, and holdfasts when waves and currents pound against them. Blennies are bottom-dwelling fish that feed on algae, detritus, small crustaceans, and plankton.

Some blennies demonstrate interesting nesting behaviors. Male blennies attract females back to their nest site, then provide sole parental care to the eggs until they hatch. Overall blennies are highly specialized fish uniquely adapted to the rugged intertidal zone.

How to Identify Small Fish

Use Size as an Initial Guide

When attempting to identify a small fish, the first characteristic to examine is its size. Generally, fish under 5 inches in length are considered small. This includes young fish that have not fully grown as well as fish species that never surpass a tiny stature, such as gobies or blennies.

It’s important to understand the difference between small adult fish versus juvenile fish. For example, a 6-inch rainbow trout is small, but it may just be a young fish that will eventually grow much larger.

Identifying features like coloration, fin shapes, and body profile will help determine if it’s an adult or baby.

Examine Physical Characteristics

Once you have established that the fish is less than 5 inches long, closer examination of physical features is needed for proper identification. Pay attention to details such as:

  • Body shape – Is it laterally compressed like a sunfish or eel-like?
  • Color patterns – Does it have spots, bands, or solid coloration?
  • Fin shapes and placement – Are the dorsal and tail fins connected or separate?
  • Mouth position – Does it face forward or downward?
  • It’s useful to have an identification guide or fish database handy to compare your observations against known species. Unique identifiers like a long dorsal fin or tiny mouth can quickly point to the correct small fish species.

    Consider Habitat and Location

    Finally, take into account where the small fish was found. Certain species prefer specific environments and geographic areas. For example:

    Fish Type Common Habitats
    Gobies Rocky reefs, tidepools
    Darters Freshwater streams, rivers
    Blennies Coral reefs, seagrass beds

    If you caught a 2-inch fish with large pectoral fins and two dorsal fins hiding in a Pacific tidepool, you could surmise it’s a tidepool sculpin rather than a darter or blenny based on geography and habitat alone.

    Identifying small fish takes patience and an eye for detail. But armed with size estimates, visible physical features, and location information, even fish biologists can pin down the species of tiny marine and freshwater creatures.


    In conclusion, small fish encompass a diverse range of species perfect for home aquariums and vital components of aquatic ecosystems. While challenging to identify at times, armed with the knowledge of key traits, habitat, and taxonomy, one can successfully classify the many small fish found throughout the world’s waters.

    We hope this comprehensive guide gave you a deeper understanding of what is a small fish called. Let us know if you have any other questions!

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