How Many Fish Can Be In A 20-Gallon Tank? Best Fish & Stocking Ideas

    Having trouble stocking a nano tank? Wondering how many fish can be in a 20-gallon tank? It’s common even for experienced fishkeepers to struggle when space is limited. There are just so many pet fish species to choose from!

    You can, theoretically, keep twenty 1-inch fish, or ten 2-inch fish, in a 20-gallon tank. This stocking strategy follows the 1 inch of fish per gallon of water rule.

    Nano fish for nano tanks, right? It’s an easy guideline to follow, but you shouldn’t apply it when stocking fish any larger than 3-4 inches.

    We’ll get into tank mate compatibility and behavior patterns to consider when choosing fish. You’ll also find out how live plants can help you make the most out of the limited space in a 20-gallon tank.

    Factors That Influence How Many Fish You Can Keep In A 20-Gallon Tank

    How many fish can be in a 20 gallon tank

    The “1 inch of fish per gallon of water” guideline is somewhat flawed. But when it comes to stocking a 20-gallon tank with nano fish (up to 3 inches long), it’s actually accurate.

    This rule of thumb doesn’t apply to larger-bodied fish, as you can’t expect a 15-inch fish to share a 20-gallon tank with five 1-inch fish.

    Even your tank setup will influence how many fish you can keep and which fish combos are ideal in a 20-gallon tank.

    You’ll need to take several factors into consideration:

    • Water quality – more fish = more waste;
    • Oxygenation – a planted 20-gallon tank will make a better home for twenty 1-inch fish than a bare one
    • Tank dimensions – a wider and shallower tank works best for a higher stocking capacity.

    If you want to stock your aquarium with multiple species, other subjective factors will come into play, such as:

    • General compatibility between different fish species;
    • What part of the aquarium they tend to occupy;
    • How they interact with males of other species;
    • Territoriality/aggression levels;
    • Shyness;
    • Shoaling behavior.

    Of course, there are common-sense criteria to consider when choosing tank mates for any aquarium. Make sure the species you’re stocking have the same preferences in water conditions.

    You shouldn’t keep tropical fish with cold-water fish. Just like you shouldn’t keep a predator in the same tank with tiny bite-sized tank mates. A recipe for disaster!

    Best Fish To Keep In A 20-Gallon Tank

    Best fish to keep in a 20 gallon tank

    To stock a 20-gallon tank, you’ll need to choose peaceful community fish. Your nano tank does limit your options, but there are plenty of spectacular nano fish to choose from.

    They can put on a show with their shoaling behavior or animate your aquarium with their vibrant colors. You’ll be amazed at how multidimensional a nano tank can become with the right setup and carefully planned fish combos.

    These are 9 of the best fish you can keep in a 20-gallon tank:


    how many fish per gallon
    Cardinal tetra (Paracheirodon axelrodi)

    Tetras are among the most popular nano fish that you can keep in a 20-gallon tank. They do best in groups of at least 6 individuals, but they often swim in schools of 12 or more.

    Tetras (spp.)

    Care level Easy
    Size 1.5 inches
    Diet Omnivorous
    Temperament Peaceful
    pH 6.8-7.8
    Temperature 75-80°F
    How many tetras can you keep in a 20-gallon tank?


    Up to 20


    how many guppies in a 20 gallon tank

    Guppies are a real visual treat in a heavily planted 20-gallon tank. Stock your nano aquarium with fancy guppies, and you’ll have a hard time looking away from them!

    Because they are livebearers, you’ll need to keep the 1:2 male to female ratio in mind.

    Males can harass females so divided attention from one male to 2-3 females, will reduce stress levels.

    Guppies (Poecilia reticulata)

    Care level Easy
    Size 1.4-2.4 inches
    Diet Omnivorous 
    Temperament Peaceful
    pH 6.8-7.8
    Temperature 72-78°F
    How many guppies can you keep in a 20-gallon tank?




    mosquito rasbora
    Mosquito Rasbora (Boraras brigittae)

    These peaceful bullet-like fast swimmers make a great stocking option for a 20-gallon tank.

    Rasboras appreciate swimming in groups of 6 or more, as they like the safety in numbers.

    If you’re planning on a single-species nano tank, you can keep up to 20 rasboras together.

    Rasboras (spp.)

    Care level Easy
    Size Up to 2 inches
    Diet Omnivorous
    Temperament Peaceful
    pH 6.8-7.8
    Temperature 75-80°F
    How many rasboras can you keep in a 20-gallon tank?




    fish for a 20 gallon tank
    Tiger barb (Puntigrus tetrazona).

    A school of tiger barbs, or rosy barbs, can be the centerpiece of a nano tank, as they’re quite standoffish. They’re fast swimmers and will prefer hanging out near the top of the aquarium.

    Keep barbs in groups of 6 individuals, and avoid having them share a 20-gallon tank with long-finned tank mates. They can get nippy, especially with slower swimmers.

    Barbs (spp.)

    Care level Easy
    Size 2.8-3.9 inches
    Diet Omnivorous
    Temperament Semi-aggressive
    pH 6.0-8.0
    Temperature 77-82°F
    How many barbs can you keep in a 20-gallon tank?




    celestial pearl danio
    Celestial pearl danio (Danio margaritatus)

    Danios are extremely active fish, so you’ll need to set up your 20-gallon tank to accommodate their needs. You should leave plenty of open swimming areas for them to explore. 

    They tend to occupy the center to upper level of the tank, and they need to be in groups of 6 or more to thrive.

    Danios (spp.)

    Care level Easy
    Size Up to 2 inches
    Diet Omnivorous
    Temperament Peaceful
    pH 7.0-7.8
    Temperature 70-78°F
    How many danios can you keep in a 20-gallon tank?




    how many bettas in a 20 gallon tank


    Betta fish are slightly trickier to pair with compatible tank mates, and you can only keep 1 male in a 20-gallon tank. Definitely avoid keeping bettas with notorious fin nippers.

    Keeping a sorority of 3 female bettas is another option that lessens the chances of fights for territory.

    Siamese fighting fish (Betta splendens)

    Care level Easy
    Size 2.25 inches
    Diet Carnivorous 
    Temperament Semi-aggressive
    pH 6.5-7.5
    Temperature 75-80°F
    How many bettas can you keep in a 20-gallon tank?


    1 male OR 1-3 females

    Pygmy Corys

    Pygmy cory catfish (Corydoras pygmaeus)
    Pygmy cory catfish (Corydoras pygmaeus) Photo Credit: Carnat Joel (Flickr)

    Peaceful bottom dwellers by nature, pygmy cory catfish are compatible with most tank mates in a 20-gallon tank.

    Keep them in groups of 6-8 to put together the ultimate algae cleaning crew in a planted nano tank.

    Pygmy Cory Catfish (Corydoras pygmaeus)

    Care level Easy
    Size 2.5 inches
    Diet Omnivorous
    Temperament Peaceful
    pH 7.0-8.0
    Temperature 74-80°F
    How many corys can you keep in a 20-gallon tank?




    How many mollies in a 20-gallon tank

    The average molly fish will grow to a size of 3 inches. Keeping three female mollies and one male molly fish together is the best option in a 20-gallon tank.

    They are livebearers, so keep an eye out for babies on a regular basis.

    Common Molly (Poecilia sphenops)

    Care level Easy
    Size Up to 4.5 inches
    Diet Omnivorous
    Temperament Peaceful
    pH 7.5-8.5
    Temperature 72-78°F
    How many mollies can you keep in a 20-gallon tank?




    how many platies in a 20-gallon tank
    Wagtail platy (Xiphophorus macalatus)

    There are so many types of platies, with distinct fin shapes, colorations, and patterns, that you can basically put together a rainbow of platy fish in your 20-gallon tank.

    Because they are livebearers, like mollies and guppies, you’ll need to make sure you’re stoking the tank with more females than males.

    Platies (Xiphophorus spp.)

    Care level Easy
    Size 2-3 inches
    Diet Omnivorous
    Temperament Peaceful
    pH 6.8-8.0
    Temperature 70-82°F
    How many platies can you keep in a 20-gallon tank?



    Popular Fish Combos To Stock A 20-Gallon Tank

    Nano fish that grow to a size between 1 to 3 inches as adults make awesome stocking options for a 20-gallon tank. Choose peaceful community fish when combining different fish species.

    Highly territorial or aggressive fish, like the pea puffer, can be unsuitable tank mates in such a small aquarium, even if they’re only 1.4 inches long.

    AqAdvisor recommends that you stock your tank to 85% of its capacity, at most. This allows your pet fish to thrive, rather than just survive, in captivity.

    Combo #1

    • 1 Betta;
    • 6 Pygmy Corydoras;
    • 8 Platies (more females than males, M-F 1:2 ratio)

    Combo #2

    • 6 Endler’s Livebearers (more females than males, M-F 1:2 ratio);
    • 6-8 Tetras;
    • 6 Pygmy Corydoras;

    Combo #3

    Combo #4

    • 8 Tetras;
    • 6 Guppies (more females than males, M-F 1:2 ratio);
    • 6 Pygmy Corydoras.

    Combo #5

    • 6 Barbs;
    • 1 male + 5 female Mollies;
    • 6 Pygmy Corydoras.

    Combo #6

    • 6-8 Platies;
    • 1 male + 5 female Swordtails;
    • 6 Pygmy Corydoras.

    Combo #7

    • 9-10 Rasboras;
    • 6 Tetras;
    • 6 Mollies (more females than males, M-F 1:2 ratio).

    Here’s a visual of how stocking a planted 20-gallon tank, with a mix of fish species, can look like:

    Fish You Shouldn’t Keep In A 20-Gallon Tank

    As you can imagine, there are plenty of fish species that are not meant to be kept in a 20-gallon tank. The reasons vary between their size, their territorial behavior, or their predatory nature.

    You can’t stock a tank with larger-bodied fish, like cichlids, following the 1 inch of fish per gallon rule. This rule of thumb is also flawed when it comes to messier fish, with a larger bioload, like goldfish.

    Here’s a list of aquarium fish you shouldn’t keep in a 20-gallon tank:

    • Angelfish

    You’ve maybe seen Angelfish juveniles in a 20-gallon tank, but adults should never be kept in nano tanks. These freshwater beauties need at least 55 gallons to thrive.

    The tank’s capacity should exponentially increase if you want to keep it with other tank mates. They are territorial with a semi-aggressive temperament and can fatally wound smaller fish.

    • Oscars

    Oscar fish can grow to an adult size of 10 inches, making a 20-gallon tank extremely unsuitable for it. Adult Oscar fish need a 75-gallon for one individual and at least a 100-gallon capacity if you’re keeping a pair of them.

    Their territoriality and aggressive nature make them incompatible with most fish species that would do well in a 20-gallon tank.

    • Common Plecos

    Although the common pleco isn’t particularly aggressive towards other fish, its sheer adult size will intimidate most fish.

    They can grow to a length between 15-24 inches, meaning that a 20-gallon tank is out of the question.

    Common plecos also have a fast growth rate, so even as juveniles, they can take over a smaller tank in no time.

    • Goldfish

    Goldfish, although you might not realize it, are large-bodied fish. It’s a myth that they only grow as large as their tank allows them. They actually face a premature death when kept in too small of an aquarium.

    They are also considered messy fish, with a bioload that can easily overpower a 20-gallon tank’s biofiltration system.

    Water quality can drop to dangerous levels fast, as they are notorious water polluters.

    Best Live Plants For A 20-Gallon Tank

    Best live plants for a 20 gallon tank

    There’s less room to “play” when it comes to aquascaping a 20-gallon tank, but a heavily planted nano tank is mesmerizing.

    Live plants will also improve water quality and oxygenation, aiding in stabilizing the biological balance inside your tank.

    Plants can help minimize fish stress/nervousness, giving them room to hide and rest when needed. In a 20-gallon tank, aquatic plants will divide the space and make for a much more peaceful cohabitation among tank mates.

    Choose easy-maintenance plants that don’t require CO2 supplements, and have low light needs, for a budget-friendly 20-gallon setup.

    Here’s a list of popular live aquarium plants that do well in nano tanks:

    • Amazon Sword

    Low-maintenance, and flourishing with even the most basic plant care, the Amazon Sword is a great choice for a nano tank.

    Substrate, diurnal exposure to light (8-10 hours per day), along with fertilizer tabs, will keep this live plant happy. 

    Its wide leaves do tend to get narrower when submerged into water, but the aesthetics of it are still amazing.

    • Vallisneria

    Planting Vallisneria in your 20-gallon tank is the easiest way to get your nano aquarium looking like a mini aquatic jungle.

    It thrives on liquid or tablet fertilizers and has a tendency to take over the underwater landscape, as it can propagate via side shoots.

    • Java Fern

    This live plant is a great choice if you want to skip the substrate/gravel when setting up a 20-gallon tank. It grows out of a rhizome, so you can keep it in place by attaching it to driftwood or rocks.

    Its roots will eventually wrap around its planting site, so it’s easy to move when aquascaping.

    Java fern has long ridged leaves and typically grows as a mat once its roots get established.

    • Dwarf Sagittaria

    Dwarf Sagittaria has a grass-like appearance and can even be kept as a carpet plant in a nano tank.

    The trick with keeping this live plant short and dense is to offer it high light (1 watt per liter or more).

    You can supplement it with nutrients in both tablet or liquid form.

    It’s an awesome low-maintenance live plant, especially if you’re planning to use the 20-gallon tank as a breeding/nursery tank.

    • Water Wisteria

    Water Wisteria bunches make great hiding/rest spots for fish that have to share a 20-gallon tank with lots of tank mates.

    It’s easy to care for and can even be planted to create a carpet in the middle/background of a nano tank.

    How To Set Up A 20-Gallon Fish Tank

    how many tetras in a 20-gallon tank
    Neon tetra (Paracheirodon innesi)

    Once you decide what type of fish you’ll be keeping in your 20-gallon tank, it’s time to work on the tank setup. You’ll need to consider the needs of your pet fish when planning out their new home. Research the fish species you’re set on, before setting up the aquarium.

    Generally, your aquascaping & tank setup should include:

    • plenty of hiding spots for shy/nervous fish;
    • an appropriately sized filter, if you’re stocking the tank up to 85% of its capacity;
    • substrate, if you’re planning on setting up a planted nano-tank.

    Here’s a list of basic gear needed for a 20-gallon tank:

    • 20-gallon Tank Kit

    A 20-gallon aquarium kit is an awesome choice if you want a plug & play nano tank.

    With plenty of filtration media on board, you can use a bacteria starter to cycle the aquarium.

    You’ll be able to begin stocking the tank as soon as you set it up.

    • Light fixture for a planted 20-gallon tank

    A full-spectrum LED aquarium light will give your live plants plenty of exposure to light, keeping your aquatic garden thriving. You’ll also need to fertilize live plants using fertilizer tabs. With just some basic TLC, your planted nano tank will reward you with a luscious view!

    • Substrate for plants

    If you’re setting up a planted tank, a high-quality substrate will make it easier for your plants to develop a good root system. Vallisneria, and carpet plants, in particular, will need substrate/gravel to grow.

    • Heater for tropical fish species

    Stocking your 20-gallon tank with tropical fish? A reliable heater becomes essential gear to keep variations in water temperature at a minimum. Tropical fish do best when the water temperature stays within 75-80°F.

    • Lid for jumpers

    If you’re stocking your nano tank with notorious jumpers (bettas!), be sure to put a lid/mesh cover/hood on your aquarium. Surface feeders, labyrinth fish, and strong swimmers can all jump right out of an uncovered tank.


    How many fish you can stock a 20-gallon tank with will depend on the adult sizes of your pet fish.

    For a nano tank, fish that grow between 1 to 3 inches in length are ideal.

    When stocking a tank with nano fish, you can use the 1 inch of fish per gallon of water rule of thumb as a guideline.

    Set up a planted aquarium, stock it with a peaceful community of miniature swimmers and you’re guaranteed to be impressed by how satisfying the results can be!

    Recommended Reading


    Don’t miss out on valuable tips! Subscribe below for our newsletter and get weekly updates on newly published posts.

      We respect your privacy. Unsubscribe at any time.