What Size is a 20-Gallon Fish Tank (Dimensions)—Best Fish and Filter

What Size is a 20-Gallon Fish Tank (Dimensions)—Best Fish and Filter

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You might be wondering what size a 20-gallon is, considering most are indicated in gallons as opposed to inches and centimeters.

Well, the dimensions of a 20-gallon fish tank depends on the design, sometimes with an aquarium being long than wide, or deeper (high) than wider or long.

Even so, most standard 20-gallon size aquariums are either 24” by 12” by 16” (high) or 30” by 12” by 12” (long), with the numbers indicating length, width and height respectively.

That said, the fish tank you purchase will depend on the fish you plan on keeping, the equipment you are going to use, and whether you’ll add live aquarium plants or not. Plus what and how much decorations you intend on having in your aquarium.

Usually, active, fast-swimming fish requires a longer tank as opposed to a higher aquarium. Whereas fish that love low lighting will do better in a deeper tank because the light is largely diffused before getting to the fish.

When adding taller background plants, you’ll want a higher aquarium, but get a longer tank if you plan on having more carpet and foreground plants, which are generally shorter.

As for equipment, a longer aquarium will do well if you want an air pump in the tank, and you have more space for a hang-on-back filter.

For a higher 20-gallon fish tank, consider using a canister filter, and especially go higher if you plan on adding LED or efficient grow lights since they have better light depth.

Which are Good Fish for a 20 Gallon Fish Tank?

Keeping fish in a 20-gallon aquarium is pretty easy since most small to medium size bodied fish will fit in there comfortably.

However, it will depend on how many you plan on maintaining.

Plus if you want a community, you may have to go for a slightly bigger tank. Same for when you want to keep schooling species.

Usually, a small to medium-sized fish that will fit in a 20-gallon will range from zero to around 4.5 inches and sometimes up to 6 inches.

Good fish for a 20-gallon fish tank include:

#1— Mollies

Mollies (Poecilia sphenops, Poecilia latipinna, Poecilia velifera) are popular livebearers and prolific breeders native to South East and South USA and Mexico.

They prefer hard water in a planted aquarium and are impressively tolerant to elevated salt levels. Mollies are also peaceful, meaning you can maintain them in a community tank.

They are omnivores and are barely fussy, although they prefer a diet that is rich in algae.

Molly fish grow to a range anywhere from 1 inch to 5 inches and will comfortably live in a 20-gallon tank. However, exert some caution when keeping sailfin mollies (Poecilia latipinna) breeds as they grow a broad and large caudal fin.

  • Size: 3.5 inches (male), 4.8 inches (female)
  • Minimum Tank Size: 20 gallons (larger breeds may require 30 gallons)
  • Care Level: Easy to moderate
  • Temperament: Peaceful
  • Tank Conditions: 68°F to 82°F, hardwater (10 to 20dGH), ph 7 to 8

#2 — Platies

Platies are another popular livebearers option closely related to the mollies. They are native to the East coast of Central America and Southern Mexico.

The fishare hardy, colorful, and make a good addition to community aquariums. Platies are also easy to breed with new color varities from extensive selective breeding.

They are also easy care for and a good choice for both expert and new aquarists alike.

Male platies grow to about 1.6 inches, while females average 2.5 inches meaning they can easily be maintained in a 20-gallon fish tank.

These livebearers are pretty tolerant to a wide range of water conditions, but look out for sudden changes in water parameters as it can lead to stress.

Like mollies, they prefer hard water and adore planted fish tanks with plenty of hiding spaces, but also enough swimming areas.

Platies are considered omnivores, but they have a substantial need for herbivorous food.

  • Size: 1.5 inches (male), 4.8 inches (female)
  • Minimum Tank Size: 20-gallons
  • Care Levels: Easy
  • Temperament: Peaceful
  • Tank Conditions: 70°F to 75°F, hardwater (10 to 20dGH), ph 6.8 to 8

#3 — Pearl Gourami

Gouramis are a popular family of fish and one of the stunning type is the pearl gourami. They have a beautiful collection of white spots across their body and large delicate fins all of which make the fish very attractive in an aquarium.

Even so, it is crucial that you have a little experience before adding pearl gourami to your 20-gallon tank.

Gouramis have labyrinth organs, so they are adaptable to low-oxygen tanks, plus they are hardy and will tolerate a wide range of water conditions albeit a little sensitive to water ph.

Generally, the fish are peaceful (with rare cases of nipping) and grow to a maximum size of 4 to 5 inches, consequently they’ll live anywhere from a 20-gallon to 30-gallon tank size.

Should you be weary of maintaining a pearl gourami fish in a 20-gallon aquarium, consider other smaller types like the dwarf gourami or honey gourami.

Try as much as possible to only maintain your pearl gourami in a heavily planted aquarium; they are a sucker for live plants.

The fish are not picky eaters, and mostly live an omnivorous lifestyle. Therefor, offer them a diet rich in both meaty and plant-based foods.

  • Size: 4 to 5 inches
  • Minimum Tank Size: 20 to 30 gallons
  • Care Level: Moderate
  • Temperament: Peaceful
  • Tank Conditions: 75°F to 82°F, hardwater (5 to 15dGH), ph 6.8 to 8

#4 — Zebra Danio

Zebra danios, also called zebrafish or danio rerio are freshwater fish belonging to the minnow family and native to South Asia.

The fish get their name from the presence of five uniform and pigmented horizontal stripes on the side of their bodies that are reminiscent to those of a zebra.

Danio rerios grow to an average of 2.5 icnhes meaning you can easily maintain them in your 20-gallon. However, they are a schooling species that need to be maintained in a group.

The fish are also not demanding and will survive in a wide range of water conditions, plus they are peaceful, so they can be kept in a community aquarium.

Moreover, zebrafish are impressively tolerant to cooler water, hence will survive in a heaterless tank and make good goldfish tank mates.

Zebra danios are not what you’d call fussy eaters and will devour nearly anything that you offer them.

A high quality flake food forms a good base diet with occasional servings of live or frozen foods.

  • Size: 2.5 inches
  • Minimum Tank Size: 10 gallons
  • Care Level: Easy
  • Temperament: Peaceful (although mildly nippy)
  • Tank Conditions: 65°F to 77°F, ph 6.5 to 7.2

#5 — Swordtails

Green swordtails are livebearers in the same Poecilia family as guppies and mollies and can live in both freshwater and brackish water.

The fish are popular in the aquarium trade and are easy to keep being a hardy species.

Natural range of swordtails are found in a North and Cental America including Mexico, Honduras, Belize, and Guatemala.

In the fish tank, swordtails like a lot of water movement and are mostly good options for a community fish. However to avoid aggression, it’s best to keep one male for every two to three females in a20-gallon tank or more.

When decorating your fish tank, keep in mind these fish are fast swimmers and pretty much like plants in their tank or rockscape to create hiding spaces.

Swordtails are omnivores, so feed them a varied diet which includes pellet, flake foods, dried worms, live food (blood worms, daphnia) nad vegetables like cucumber and zucchini.

  • Size: up to 5.5 inches
  • Minimum Tank Size: 20 gallons
  • Care Level: Easy
  • Temperament: Peaceful
  • Tank Conditons: 72°F to 82°F, hard water (10 to 15 dGH)ph 7.0 to 8.0

#6 — Tiger Barbs

Tiger barbs are a great addition to most tanks and are easy to to care for. They are also beautiful with yellow, orange, or golden-red background coloration and black stripes.

However, tiger barbs are semi-aggressive and are known nippers, but they still make good community fish and enjoy swimming in schools.

These barbs grow up to 2.7 inches in length hence are perfect for keeping in a 20-gallon set up.

Keep your tiger barbs in a group of 6 or more individuals as they are a schooling species.

Tiger are native to the malayPeninsula, sumatra, and Borneo in Indonesia,so maintain them in a tropical aquarium, layered with a substrate at the bottom and plenty of spaces for the fish to swim around.

Tropical flake, brine shrimp and other small crustaceans, daphnia, glass worms and blood worms are all good food options for tiger barbs. Feed them twice a day if you can, or once a day.

Plus be sure to use a high quality flake food which mostly make the base meal for most aquarium species.

  • Size: 2.8 to 3.9 inches
  • Minimum Tank Size: 20 gallons
  • Care Level: Easy
  • Temperament: Aggressive
  • Tank Conditions: 68°F to 80°F, water hardness (5 to 19 dGH), ph 6.0 to 8.0

#7 — Dwarf Cichlids

As opposed to all the other 20-gallon species here, dwarf cichlids are a group of different small cichlid fishes rather than a single specimen.

They include Kribensis, german blue ram cichlid, blockhead cichlid and more. For this reason, care and maintanance will vary from one species to the other, though most will do OK in a 20-gallon fish tank.

A good rule of thumb when stocking dwarf cichlids is to replicate their natural habitat, mostly akin to the tropical regions (Africa and South America) where majority come from.

  • Size: The suggested maximum length is 4 inches
  • Minimum Tank Size: 20 gallons
  • Care Level: Moderate to Hard
  • Temperament: Largely Peaceful
  • Tank Conditions: Varys depending on the species and natural habitat

#8 — Kuhli Loach

Kuhli loach are also commonly called coolie loaches are native to the tropical waters of Indonesia, Malaysia, Java, and surrounding areas.

They are unique bottom-dwelling fish with bodies that are eel shaped with a color that is kind of salmon-pink or yellow and dark brown to black stripes that half circle the body.

Coolise are rather tiny loaches and fairly easy to care for in a home aquarium, and only need water that is medium-soft and a ph around neutral.

Usually, kuhli loaches will grow to an average of 3 to 5 inches in length hence are good candidates for a20-gallon aquarium, plus they have a light bioload.

Even so, rememeber to use a sand or soft stones substrate since these are bottom-dwelling eel-like fish and may scartch their bodies.

Also avoid adding other sharp decorations in your fish tank.

Coolie loaches are scavengers and get a substantial amount of food by digging for leftovers. However, this should not be taken to mean they can survive on scraps, so supplement their diet with sinking food pellets and assorted live foods.

  • Size: 3 to 4.5 inches
  • Minimum Tank Size: 20 gallons
  • Care Level; Easy to Moderate
  • Temperament: Peaceful
  • Tank Conditions: 75°F to 85°F, ph 6 to 7

#9 — Yoyo Loach

Yoyo loaches, almora loaches or Pakistani loaches are another freshwater species in the loach family that are perfect for a20-gallon aquarium.

They are native to slow-flowing and still waters of the Ganges basic in Northern India and possibly Nepal.

The loaches are largely peaceful, though they are known to fight with members of their own species.

For this reason, yoyo loaches do well with other peaceful fish and can even stand their own with more aggressive fish in the tank.

Pakistani loaches occupy the bottom and sometimes middle water columns, but never come to the surface. They are hardy fish and will tolerate a wide range of water conditions, though they require reasonably stable parameters.

As they like slow-moving water, you can either use an under gravel filter or canister filter in your aquarium which will oxygenate the tank and keep it clean.

That said, a 20-gallon fish tank will work for young yoyo loaches, but you may need at least a 40-gallon when the fish get to adult size.

For each yoyo in your aquarium, add 10 to 15 gallons of water, which means your 20-gallon can only accomodate a single fish.

  • Size: up to 2.5 inches
  • Minimum Tank Size: 20 gallons
  • Care Level: intermediate
  • Temperament: Peaceful
  • Tank Conditions: 75°F to 86°F, Hardness (3 to 10 dGH), ph 6.5 to 7.5

#10 — Killifish

Killifish are vibrantly colored and beautifully patterned freshwater fish and are one of the more popular species among home aquarium owners.

Moreover, they are many species of killifish meaning you will get almost any coloration and finnage you desire.

The most popular aquarium species have two different habitats.

There are those native to areas of tropical rainforest and inhabit pools, swamps, and streams, and the others (more popular) live in ponds inside the tropical savannahs.

Even so, all killifish are small in size even in the wild, so you can rest assured they will fit in your 20-gallon fish tank.

They prefer live foods in the aquarium, but can also live on flake food supplemented with frozen brine shrimp and blood worms.

However, although they are temperamently fine for community tanks, killifish dietary needs and preference for cooler temperatures make them a little challenging to companion.

  • Size: 0.7 to 3.5 inches
  • Minimum Tank Size: 20 gallons per pair
  • Care Level: Easy to Diffucult (depending on the species)
  • Temperament: Mostly peaceful
  • Tank Conditions: Depends on the species

How Many Fish Can Be in a 20-gallon Tank?

Unfortunately, there is no direct answer to this question because it all depends on the species you plan on keeping. Adding 10 two-inch guppies to a tank is not the same as dropping in two 10 inch fish.

Having said that, a good rule of thumb is to have a gallon of water for every inch of fish you plan on keeping. But still, there are variable you’ll need to be wary about.

For starters, some fish species are quite messy while others have a hearty appetite hence a large bio-load. This means if you maintain more than of such fish, you will have major water quality issues.

Ergo, even if they fit in your 20-gallon aquarium, it is not advisable.

Second, other fish like Zebra danios, tiger barbs, tetras, and corydoras need to be maintained in schools. Consequently, while a single or a pair will fit in your small fish tank, you’ll need more space to keep the appropriate number.

Therefore, when stocking your 20-gallon fish tank, research each of these species individually to determine how many of each will fit in your aquarium.

What is The Best Filter for a 20-gallon Fish Tank (Aquarium)?

When setting up your 20-gallon fish tank, a good filtration system is part of the things you’ll need to take into consideration. Of course, you’ll also want to invest in an appropriate air pump, thermometer, heater, substrate, and lighting.

That being said, there are several types of filter you could use, but if you are a beginner, an under gravel filter is a great option for your 20-gallon aquarium.

It is cheap and efficient given the size of the tank and is fairly basic to operate and set up as well.

Alternatively, you can go with a box filter (corner filter) which is also fairly easy to use, though it would be more appropriate if you had a 10 gallon fish tank.

Even so, if you plan on maintaining messy or outrightly dirty fish, a power filter offers better chemical, biological, and mechanical filtration. Plus it is easy to set up, maintain, and use and most are fairly priced.

Canister filters are probably some of the most effective filters in the aquarium trade, but they are best for larger tanks with bigger fish. Should you use it for your 20-gallon, you’ll will get the best filtration, but the setup will be grossly out of scale.

Also, canister filters are a pain to take apart, especially for new hobbyists.

Hang-on-back filters will work fine, and many have sponges for your nitrification bacteria to grow on, though they hang drag at the back and risk denting your aquarium aesthetics.

Either way, make sure the filter you use is rated for at least double your aquarium capacity particularly if you will have fish with a heavy bioload or messy tendencies.

I recommend you go with a 30 to 50 gallons rated filter with adjustable flow, so you can turn it down to keep it from causing a storm, but still filter your tank adequately.

Lastly, you’ll want to purchase your equipment from a reputable manufacturer like fluval or Aquaclear (albeit a little pricey) to be assured of quality and efficiency.

Some good filtration systems for a 20-gallon fish tank include:

  1. Aquaclear 20, 30, 50 power filter: It is a unique multistage filter system that provides complete filtration, is easy to clean, and relatively quiet. However, you will need a bit of space behing your aquarium for the fixture.
  2. Marineland penguin filter w/multistage: This filter uses the Rite-size B filter cartridage to deliver three-stage filtration. Even so, old penguin filters are nice, but you might not become an instant fan of the newer models as they “sort-of” tend to get noisy after a short while.
  3. Fluval C2 power filter: While a 30-gallon filter rating is not much, this system can process around a 120-gallons of water per hour, meaning it can handle more than 4 times the total amount of your 20-gallon aquarium in an hour. Plus it has an impressive 5-stage filtration and does engage all three major forms of filtration.
  4. Aqueon quiet flow LED pro 20,30,50: This filter features an LED cartridge change indicator light that flashes when the cartridge becomes clogged with debris, which is especially important for beginner fish keepers. It is also has an internal pump design, so it is pretty quiet. A self priming setting aid the filter to start up automatically after cleaning or power interruptions and it is also a 5-stage, high flow rate system.
  5. Penn plax cascade canister filter: The swimming pool style rotating valve is one thing that explains it’s efficiency. It is made for an aquarium that is up to 30-gallons and comes with flow-control.

That’s all! Enjoy your 20-gallon aquarium

Eddie Waithaka

Resident Content Creator and Marketer at AquariaWise who talks about aquariums and fish and aquascapes a lot.

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