What Fish Can Live in a 20-Gallon Tank? 16 Species Compared

The short answer

Sources name small schooling fish, corys, honey gouramis and livebearers; Aquarium Co-Op picks harlequin rasboras, leopard danios and panda corys (LiveAquaria lists 30 gallons for pandas). Aqueon says 20- to 50-gallon tanks hold the small fish of smaller tanks, “even more of them”. Sources differ on rams, tiger barbs, pearl gouramis and goldfish; betta figures depend on tank mates.

FishBase, a scientific fish database, gives lengths and water ranges in metric; figures after “≈” are our conversions. Aqueon is an aquarium-products maker; LiveAquaria (LA) and Aquarium Co-Op (Co-Op) are retailers. All figures are for freshwater. See also our guides to fish for a 10-gallon tank and beginner fish.

The 9 Species at a Glance

FishBase’s “Max length” is the largest length it lists, not a typical adult size. It gives some as SL, which ends at “the midlateral posterior edge of the hypural plate”, the “base of the caudal fin”, and some as TL, “the greatest length of the whole body”. LA and Co-Op do not say which length they give.

Species	Size	Tank size, per source
Harlequin rasbora	5.0 cm TL ≈2.0 in (FishBase); 2 in (LA)	10 gal (LA); “10-gallon aquariums or larger” (Aqueon)
Neon tetra	2.5 cm SL ≈1.0 in (FishBase); 1.5 in (LA)	10 gal (LA); 10-20 gal for most tetras (Aqueon)
Zebra danio	3.8 cm SL ≈1.5 in (FishBase); 3 in (LA); 2 in (Co-Op)	10 gal (LA)
Celestial pearl danio	2.1 cm SL ≈0.8 in (FishBase); 1 in (Co-Op)	10 gal; 20 gal or larger for a school of 10-15 (Co-Op)
Cherry barb	5.0 cm TL ≈2.0 in (FishBase); 2 in (LA)	10-20 gal (Aqueon); LA: 20 gal in its stats, 30 in its text
Panda cory	3.8 cm SL ≈1.5 in (FishBase); 2.5 in (LA)	30 gal (LA); 6+ in 20 gal (Co-Op)
Honey gourami	13.7 cm TL ≈5.4 in (FishBase, from a wild-fish survey); 2 in (LA); “around 2 inches” (Co-Op)	10 gal (LA, Aqueon); 3 “do better” in 20 (Co-Op)
Guppy, platy	Guppy 5.0 cm SL ≈2.0 in; platy 4.0 cm TL ≈1.6 in (FishBase)	5-20 gal for most livebearers (Aqueon)

FishBase’s honey gourami figure comes from a length-weight survey of wild fish in an Indian reservoir (its Ref. 125929); FishBase’s own dwarf gourami figure is 9.5 cm TL.

Water Ranges by Source

Aqueon gives one range per group (most tetras, most livebearers).

Species	FishBase: pH; temperature	Maker and retailers: pH; temperature
Harlequin rasbora	5.0-7.0; 22-25°C ≈72-77°F	Aqueon (commercially raised): 6.8-7.8; 75-80°F; wild-caught pH 5.0-7.0. LA: 6.0-6.5; 72-77°F
Neon tetra	5.0-7.0; 20-26°C ≈68-79°F	Aqueon: 6.8-7.8; 75-80°F. LA: 5.5-7.0; 68-78°F
Zebra danio	6.0-8.0; 18-24°C ≈64-75°F	Aqueon: 7.0-7.8; 70-78°F. LA: 6.5-7.5; 64-75°F
Celestial pearl danio	not given	Co-Op: 6.6-8.0; “thrive” at 72-76°F, live “up to 78°F”
Cherry barb	6.0-8.0; 23-27°C ≈73-81°F	Aqueon: 6.8-7.8; 75-80°F. LA: 6.0-7.0; 74-79°F
Panda cory	6.0-8.0; 20-25°C ≈68-77°F	Aqueon (captive-bred corys): 7.0-8.0; 74-80°F. LA: 5.8-7.0; 72-79°F. Co-Op: 72-77°F
Honey gourami	6.0-8.0; 22-28°C ≈72-82°F	Aqueon: 6.8-7.8; 75-80°F. LA: 6.0-8.0; 72-78°F. Co-Op: 6.0-8.0; 74-82°F
Guppy, platy	Guppy 7.0-8.0; 18-28°C ≈64-82°F. Platy 7.0-8.0; 18-25°C ≈64-77°F	Aqueon (most livebearers): 7.0-8.4; 74-78°F. Co-Op, fancy guppies: 76 to 78°F

The Species, One by One

Harlequin Rasbora (Trigonostigma heteromorpha)

Groups: LA says “schools of 8-10 individuals”, Aqueon “groups of 7 or more”, Co-Op “a group of six to eight”, FishBase “groups of 5 or more”. LA rates it “Care Level: Easy” and “Temperament: Peaceful”.

Neon Tetra (Paracheirodon innesi)

LA says “schools of six or more” and calls it “Extremely peaceful”; Aqueon says tetras “do best in schools of 6 or more”.

Zebra Danio (Danio rerio)

Aqueon says danios “are best kept in schools of at least 6 or more”. Co-Op picks the zebra danio’s “cousin”, the leopard danio, for 20 gallons: “Get a group of six”. Co-Op notes that zebra danios “are very active and need more swimming room.” AQQA, an aquarium-equipment brand, lists “fast swimmers (e.g., danios)” among fish not to stock in 20 gallons: they “need long tanks for constant movement, not compact or tall tanks”. LA: “Peaceful”.

Celestial Pearl Danio (Danio margaritatus)

Co-Op: “the rule of thumb for schooling fish is to get at least six fish of the same species, but we strongly suggest buying 10-15 CPDs since they can be a bit timid”.

Cherry Barb (Puntius titteya)

Aqueon lists cherry barbs among “peaceful species” and says barbs “do best in schools of at least 6 to 10 fish”.

Panda Cory (Corydoras panda)

Tank size: Aqueon calls panda corys “good choices” in its 5- to 10-gallon section, and its Corydoras guide says “A 30-gallon aquarium is best for most species”. Bubble-Magus, an aquarium-equipment maker, lists “1-2 peaceful corydoras catfish” for 20 gallons, against Co-Op’s “group of six or more for a 20-gallon aquarium”.
Caution: Aqueon’s Corydoras guide: “Pristine water quality is essential to good health in these catfish. They should never be added to new aquariums or those that have been neglected.” Its tank-size guide adds: “Like all bottom feeders, it is important to feed them directly and not just depend on them getting leftovers from feeding other fish.”

Honey Gourami (Trichogaster chuna)

Co-Op names no sexes for its group of three and says honey gouramis “are considered to be peaceful community fish”. Aqueon’s gourami guide: “Male gouramis have a tendency to be aggressive towards each other, so they should typically be kept individually. Female gouramis usually tolerate each other well.” AQQA’s 20-gallon community lists a honey gourami as its “centerpiece fish”.

Guppy (Poecilia reticulata) and Platy (Xiphophorus maculatus)

Aqueon advises “a ratio of 2 to 3 females per male” and warns: “if you have male and female livebearers, you will eventually have fry – plan accordingly!” For guppies Co-Op says “a 10- or 20-gallon aquarium would be more appropriate in the long run” because they breed quickly.

Where Sources Differ on 20 Gallons

Fish	For 20 gallons	Against, or larger
German blue ram	Co-Op: “one male with one or two females”; LA text: “at least 20 gallons”	LA stats: 30 gal, “Expert Only”. Temperature: Co-Op “ideally between 84-86°F”; LA 72-79°F; FishBase 27-30°C ≈81-86°F
Swordtail	LA: “twenty gallons or more”, 4 in; Aqueon lists it for 20-50 gal	FishBase: 14.0 cm TL ≈5.5 in; LA: males “can get territorial”
Tiger barb	Aqueon’s tank-size guide: 20-50 gal “in groups of 6 or more”	Aqueon’s barb guide and LA: 30 gal; LA: “may pester or even nip the fins of larger, slower moving fish”
Pearl gourami	Aqueon’s tank-size guide lists it for 20-50 gal	Aqueon’s gourami guide and LA: 30 gal; LA: 4.5 in; FishBase: 12.0 cm TL ≈4.7 in
Bristlenose pleco	Aqueon, 20-50 gal: “A much better choice” than the common pleco; AQQA: “1 bristlenose pleco” in a livebearer tank	LA: 30 gal, 6 in; AQQA’s not-to-stock list: “large cichlids or plecos”, which “grow far too large for a 20 gallon tank”

Bettas and Goldfish: What Each Source Says

Betta (Betta splendens). For one betta, LA says it “should be kept singularly in aquariums of at least 4 gallons in size or with suitable, peaceful tank mates.” For a community, Aqueon: “Male bettas are highly territorial towards each other and must be kept separately or in a partitioned container. They can be kept with other fish in a peaceful community aquarium of 15 gallons or larger.” With tank mates, Co-Op asks for “at least 10 to 20 gallons with lots of cover and live plants or else the betta fish may become overly territorial.” For 20 gallons, AQQA pairs one betta with 8-10 ember tetras or harlequin rasboras and says “stay away from colorful fin-nippers like tiger barbs”; Bubble-Magus lists “1 betta (with tankmates, if compatible)”. Aqueon: “Avoid putting them with male gouramis, fin-nippers such as tiger barbs or active fish like giant danios,” and “bettas are jumpers, so keep a secure lid on the tank!”
Goldfish (Carassius auratus). Aqueon: “Goldfish should never be kept in bowls, small aquariums or any unfiltered container!” It says adult common goldfish, comets and shubunkins “should have at least 20 gallons of water per fish, while adult fancy goldfish should have at least 10 gallons per adult fish.” Its tank-size guide calls 20-50 gallons “an excellent tank size range for fancy Goldfish” and says “A 29-gallon aquarium would be a good home for 3-4 of the fancy Ryukin or Oranda varieties.” Co-Op, for one fancy goldfish: it “can potentially grow to 8 inches (20 cm) in length, so a 20-gallon long aquarium is often recommended as the minimum tank size.” AQQA answers “zero” goldfish for 20 gallons; they “really need 40 to 55 gallons at a minimum”. FishBase gives the species a 48.0 cm TL max length ≈18.9 in.

How Many Fish Fit in 20 Gallons?

Co-Op: “In the past, it was often recommended to beginners that you can keep 1 inch of fish for every 1 gallon of water. This rule of thumb mainly applies to small community fish that are approximately 1-3 inches (2-7 cm) in size.” For a cycled tank, Co-Op’s “easiest way” is “measuring the nitrate level and making sure it stays below 40 ppm”.
Bubble-Magus says the rule “may sound easy, but it’s far from the full picture”; for 20 gallons, “with a well-planned stocking list and proper filtration, you can house up to 10-15 smaller fish, or a few medium-sized species”.
AQQA: “most 20 gallon tanks can safely handle somewhere between 10 to 20 small fish. This answer depends on the species, adult size, temperament, filtration, and how heavily planted your aquarium is.”
Aqueon gives no count. For 20-50 gallons it warns that with 3-4 inch fish “sometimes their natural territoriality and aggression can be a concern, so care must still be made to not overcrowd them.” On nitrate: “Nitrate toxicity to fish varies depending on species, age and overall health, but levels above 50 ppm are undesirable in freshwater aquaria.”

Before the First Fish

Must-haves

A cycling plan

API, a test-kit maker, describes two ways: a “fishless cycle” that “would require daily dosing of ammonia and bacteria”, and a “fish in cycle” where “it is recommended to add small amounts of fish at a time”. See how to cycle a fish tank and our tank set-up guide.

A filter

Aqueon says flow should be “at least four times the volume of the tank per hour”, which is 80 gallons per hour for 20 gallons (our arithmetic). AQQA suggests “a filter rated for 30 to 40 gallons”.

A heater and thermometer

Aqueon advises a heater when the room is below 75° for rasboras, tetras and gouramis, 74° for livebearers and corys, 70° for danios. See our tank temperature guide.

A test kit and water conditioner

Aqueon says to “Test the aquarium water for pH, ammonia and nitrite before adding new fish” and “Always treat tap water with a water conditioner to neutralize chlorine and ammonia before adding it to the aquarium.” It says “The only safe ammonia level is zero” and “The only safe nitrite level is zero”. See our freshwater test kit roundup.

A slow start

Aqueon: “only buy a few fish at a time. Wait at least a week between new fish additions.” Co-Op checks nitrate “each week for 2 to 3 weeks in a row” before the next species. See our guide to adding new fish.

#	Preview	Product	Price	Link
1		Tetra 20 Gallon Complete Tropical Fish Tank Kit \| Aquarium…20-gallon kit with heater	$119.14 Prime	Check on Amazon
2		Fluval AquaClear Fish Tank Power Filter for 20- to…Filter for 20-50 gallons	$61.99	Check on Amazon

Prices as of Nov 9, 07:11 UTC

The kit is compared in our betta fish tank roundup and the AquaClear 50, rated by Fluval for 20 to 50 gallons, in our filter roundup.

What changes the list

Soft or hard tap water: FishBase gives neons 1-2 dH; LA lists neons at 4-8 dKH, and Aqueon gives most tetras “alkalinity between 3° and 8° dKH”. FishBase gives guppies and platies 9-19 dH. dH (general hardness) and dKH (carbonate hardness) are separate tests (our reading). Co-Op says rams “do better in soft water or low GH conditions”. See our checklist for choosing fish for a tank.

Ready to buy?

LA lists six of the nine as “Diet: Omnivore”; our tropical fish food roundup compares flakes, granules and sinking pellets.

Frequently Asked Questions

Q. What is the biggest fish for a 20-gallon tank?

None of the pages here gives a largest-fish limit for 20 gallons. Aqueon says that in 20-50 gallons “you can start to keep some fish that get into the 3-4” size range”, and that the common pleco “will get too large for tanks of this size and should not be kept.” LA sets the swordtail (4 in) at “twenty gallons or more”.

Q. How many tetras can I put in a 20-gallon tank?

Sources differ: Bubble-Magus suggests a “small school of tetras (8-10 fish, such as neon or rummy nose tetras)”, AQQA “around 15 to 20” neons. Aqueon and LA give six as a minimum school.

Q. What is a good centerpiece fish for 20 gallons?

Co-Op’s cory guide pairs corys and small tetras in 20 gallons with “a centerpiece fish like a honey gourami”.

Q. What should I do if a fish looks sick?

Test ammonia and nitrite first, and ask an aquatic veterinarian about the fish; disease and medication are outside what this site covers.

Sources (54)

Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon.com at the time of purchase will apply to the purchase of this product.

Aquarium Tales Team

Written and reviewed by the Aquarium Tales Team. See the About page for how we work.