Best fish for small tanks: 14 nano fish and invertebrates that actually fit

01

What makes a fish good for a small tank

Searching for the best fish for a small tank usually starts with size, but adult length is only one of four criteria. A fish that stays small can still be a poor nano tank fish if it swims fast, needs a large school or produces a lot of waste. Use these rules before you buy anything labeled "small aquarium fish" at the store.

Remember that small volumes are less forgiving. Ammonia, temperature and pH change faster in 5 gallons than in 55, so a small tank is not automatically easier for beginners. If this is your first aquarium, read our beginner aquarium guide and cycle the tank before adding any fish.

• Adult size: under about 1.5 inches (4 cm) for 10 gallons and under, under 2 inches (5 cm) for 15–20 gallons.
• Activity level: slow or mid-water hoverers fit; fast, long-distance swimmers like zebra danios need tank length.
• Group needs: schooling fish need 6–10 of their own kind, so the whole group has to fit, not just one fish.
• Bioload: goldfish and plecos produce far more waste than their size suggests and overload small filters.
• Temperament: territorial or fin-nipping species become a problem when there is no room to escape.

02

Small aquarium fish compared

The table compares every fish for nano tanks discussed on this page, including a few without their own guide yet. Minimum tank sizes are conservative and assume a cycled, filtered and heated tank. Group size is the smallest number that lets a species behave naturally.

03

Best single fish: the betta

For a 5 gallon tank, a single male betta fish is the most rewarding choice. It is colorful, recognizes its owner, does not need a school and is calm enough for a small volume. What it does need is a heater set around 78 °F (25.5 °C) and a gentle filter, because long fins struggle in strong current. A sponge filter is ideal; see our picks for the best sponge filter.

Keep one male per tank. Females can live in groups only in much larger, heavily planted tanks, and even then aggression is common, so a betta sorority is not a small-tank project. A nerite snail is the safest companion in 5 gallons; for 10 gallons and up, our betta tank mates guide lists the options.

04

Best tiny schooling fish for nano tanks

A school of very small fish makes a 10 gallon tank look larger and more natural. The ember tetra is the easiest to find and very peaceful. Chili rasboras are even smaller and show their red best in a dark, planted tank. The celestial pearl danio is spectacular but shy; it needs dense plants and does better at the cooler end of the tropical range, around 72–76 °F (22–24 °C).

Once you reach 15–20 gallons, your choice widens. The harlequin rasbora is hardy and forgiving, and the neon tetra looks best in a group of 10 or more, which is why we do not recommend it under 15 gallons. Whatever you pick, buy the whole school within a few weeks so the group bonds, and only after the tank is fully cycled.

Livebearers are a special case. Male guppies and Endler’s livebearers are colorful and hardy, but a mixed-sex group breeds constantly and will overstock a small tank within months. Keep males only. Our guppy guide explains how to tell them apart.

05

Bottom dwellers, algae eaters and invertebrates

Most popular cleanup fish are too big for small tanks. Common plecos grow past 12 inches (30 cm), and even the bronze corydoras prefers a group in 20 gallons or more. In a 10 gallon tank, choose dwarf species from the corydoras family, such as Corydoras pygmaeus or C. habrosus, on soft sand so their barbels stay intact.

Otocinclus are excellent soft-algae grazers but need a mature tank with established biofilm, at least 15 gallons and a group of four or more. Many arrive starved, so feed them algae wafers or blanched zucchini while they settle in.

For the smallest tanks, invertebrates are often the better answer. A cherry shrimp colony thrives in 5 gallons and breeds without help. A nerite snail cleans glass and decor without overpopulating the tank. If algae is your main concern, compare options in our guide to the best snails for algae.

06

Small tank stocking examples

These sample communities assume a cycled, heated tank with a filter rated for its size, live plants and weekly water changes of 20–30%. Add fish in two or three batches, a few weeks apart, so the bacteria can keep up. For tank-specific plans, see our pages on fish for a 5 gallon tank, the best fish for a 10 gallon tank and the best fish for a 20 gallon tank.

• 5 gallons (19 L), betta tank: 1 male betta + 1 nerite snail.
• 5 gallons (19 L), shrimp tank: 10 cherry shrimp to start + 1 nerite snail, heavily planted.
• 10 gallons (38 L), nano school: 8 ember tetras or 8 chili rasboras + 6 pygmy corydoras.
• 10 gallons (38 L), betta community: 1 male betta + 6 pygmy corydoras + 1 nerite snail.
• 10 gallons (38 L), livebearers: 4 male Endler’s or guppies + cherry shrimp (some shrimplets will be eaten).
• 20 gallons (75 L), community: 10 harlequin rasboras + 6 pygmy corydoras + 3 otocinclus.
• 20 gallons (75 L), planted nano: 10 celestial pearl danios + 3 sparkling gouramis + cherry shrimp.

07

Equipment for a small tank

Small tanks need the same equipment as large ones, just scaled down. Choose a heater sized for the volume with an adjustable thermostat (see the best aquarium heater guide), a filter with gentle flow, a lid, and a reliable liquid test kit. A water conditioner that neutralizes chlorine and chloramine is essential at every water change. If you have not bought the tank yet, a 10 gallon kit is the most flexible size; compare models in our best 10 gallon aquarium review.

08

Fish to avoid in a small tank

Some of the most common fish sold to new owners of small tanks are the worst fit. They may be small in the store, but they either grow large, swim fast or produce too much waste.

• Goldfish: grow to 6–12 inches (15–30 cm) and need 30 gallons or more, even fancy varieties.
• Common pleco: reaches 12–18 inches (30–45 cm). Even a bristlenose pleco needs about 20 gallons or more.
• Angelfish: tall bodies and up to 6 inches (15 cm) tall; they need 30 gallons or more.
• Zebra danios and white clouds kept warm: very active swimmers that need tank length, not just volume.
• Tiger barbs: fin nippers that need a large group to spread aggression.
• Mollies and swordtails: larger livebearers that breed fast and outgrow 10 gallons.
• Bala sharks, clown loaches, oscars: sold small, but grow to a foot (30 cm) or more.