Can I Keep An Oscar In A 20-Gallon Tank?

Disclosure: When you purchase something through my affiliate links, I earn a small commission. As an Amazon Associate, I earn from qualifying purchases.

Oscars are beautiful and interesting creatures, there is no doubt about it. But when it comes to their tank size, they can be pretty demanding.

Can you keep a single Oscar fish in a tank of only 20 gallons? In which situations you can and in which you can’t? And how much does a bigger tank cost?

In this article, you will learn everything there is to know about this topic, so you leave as an expert. Let’s get started.

Can You House an Oscar in a 20-Gallon Tank?

No, you cannot permanently house an Oscar fish in a 20-gallon tank. Only a single baby Oscar fish can temporarily reside in such a tank, but it will outgrow it rapidly.

Here are reasons to support this statement:

  • Growth Rate: Oscar fish grow quickly, reaching 12-14 inches in just over a year, which is too large for a 20-gallon tank.
  • Space Needs: Adult Oscar fish require ample space to swim, and ideally, each fish needs a minimum of 55 gallons.
  • Aggressive Behavior: When Oscar fish feel confined, they can become more aggressive, making a small tank a potential hotspot for stress and conflict.
  • Water Quality: In a small tank like 20-gallons, the waste produced by an Oscar fish can rapidly degrade water quality, demanding frequent maintenance.
  • Healthy Development: For optimal health and development, Oscar fish need space to explore, play, and establish territories.

Also Read: Oscar Fish Tank Size

Until What Age Can Baby Oscars Thrive in a 20-Gallon Tank?

Baby Oscar fish thrive in a 20-gallon tank until they’re around 6 to 8 months old. However, as they age, their space needs change, prompting the need for a larger tank.

Here’s why it’s essential for Oscar fish:

  • Speedy Growth: In just their first year, Oscars can shoot up to a size of 10-12 inches, quickly outgrowing a 20-gallon space.
  • Movement Needs: As they get bigger, Oscars need extra room to maneuver, ensuring they stay spry and robust.
  • Water Quality: A larger Oscar means more waste, challenging a 20-gallon tank to keep the water clean without regular upkeep.
  • Comfort Factor: Cramped quarters can elevate stress levels in Oscars, potentially causing health and behavioral shifts.
  • Feeding Concerns: A maturing Oscar has a heftier appetite, and a compact tank might not support the essential bio-filtration, which can influence water quality.

What Is the Minimum Tank Size for an Adult Oscar Fish?

The minimum tank size recommended for an adult Oscar fish is 55 gallons. However, a 75-gallon tank or larger is preferable to provide optimal conditions.

Here’s why the size is critical for the Oscar fish:

  • Size and Mobility: Oscar fish can reach lengths of 12-14 inches, requiring ample space for unhindered movement and swimming.
  • Water Quality: Larger tanks help in maintaining stable water parameters, essential for the Oscar fish’s health and longevity.
  • Territorial Behavior: Oscar fish are territorial, and a larger space reduces aggressive tendencies and stress.

Also Read: Can You Have An Oscar In A 10-Gallon Tank?

How Can You Determine the Appropriate Tank Size for an Oscar Fish?

Here’s how fish owners estimate the minimum 55 gallons rule:

  • Begin with the standard that fish generally require 1 gallon of water for every inch of their body length.
  • Estimate the adult size of an Oscar fish, which can grow to be 12-14 inches in length. Based on the 1-inch-per-gallon rule, this would mean 12-14 gallons at a minimum.
  • Factor in additional space for the Oscar’s active swimming behavior, effectively doubling the space needed. This brings the requirement to 24-28 gallons.
  • Account for the fish’s territorial nature and the need for space to reduce stress and aggression, adding another 20-25 gallons to the calculation.
  • Combining all these considerations, the total comes to around 55 gallons as a baseline recommendation for one adult Oscar fish.

How Much Room Do Additional Oscar Fish Need?

When adding more Oscar fish to your aquarium, allocate an extra 20 to 30 gallons of water per fish. Here’s the rationale:

  • Size Considerations: Oscars can grow up to 12-14 inches, meaning they need ample room.
  • Territory Needs: Oscars are territorial. More space can minimize clashes and aggression.
  • Swimming Needs: Oscars are lively swimmers and benefit from room to move, which keeps them healthy and less stressed.
  • Water Cleanliness: A larger water volume can better dilute the increased waste from more Oscars.
  • Tank Balance: When more Oscars join, the balance of the tank shifts, meaning you’ll need added space for hiding and territory.

Also Read: Can You Have An Oscar In A 30-Gallon Tank?

How Much Does a 55-Gallon Tank Typically Cost?

A 55-gallon tank’s average cost usually lies between $100 and $300, influenced by its brand, quality, and added features. Factors that can sway the price include:

  • Material: While glass tanks often cost more than acrylic ones, they’re typically longer-lasting.
  • Included Features: Tanks equipped with filtration systems, lighting, or heaters might have a higher price tag.
  • Brand Strength: Renowned aquarium brands may charge a bit more due to their known quality.

What’s the Cost of a 75-Gallon Tank?

A 75-gallon tank can range from $150 to $500 on average, with fluctuations based on brand, material, and included features. Several elements can affect the price:

  • Material: As with the 55-gallon tank, glass versions might be pricier than acrylic counterparts.
  • Included Accessories: A tank that comes with a stand, hood, lights, or a filtration system will typically cost more.
  • Manufacturer Reputation: Renowned brands known for quality and durability might have their tanks priced higher.

What Is the Biggest Fish for a 20-Gallon Tank?

The largest fish suitable for a 20-gallon tank would typically be around 3 to 4 inches in adult size. Oscar fish, for reference, would quickly outgrow such a space and are not suitable.

Key reasons to limit the fish size for a 20-gallon tank include:

  • Mobility Concerns: Fish that grow too large for their tank have restricted mobility, which can impact their health and stress levels.
  • Water Quality: Larger fish produce more waste, which can quickly overwhelm a 20-gallon tank’s filtration system, leading to deteriorating water conditions.
  • Growth Potential: Unlike Oscar fish which can reach lengths of 12-14 inches, fish for 20-gallon tanks should have limited growth potential to ensure they don’t outgrow their environment.

Fish species that fit well in a 20-gallon tank and grow to about 3 to 4 inches include:

  • Platy
  • Honey Gourami
  • Dwarf Gourami
  • Mollies
  • Corydoras Catfish

Which Fish Species Can I Maintain in My 20-Gallon Tank?

While a 20-gallon tank might not provide a lot of space for Oscars, it’s still suitable for several smaller fish species:

Fish SpeciesNumber in 20 Gallons
Neon Tetra15-20
Dwarf Gourami1-2
Betta Fish1 male or 5-6 females
Zebra Danio12-15
Corydoras Catfish6-8

1. Platy

Platies are colorful, lively fish that thrive in community tanks. In a 20-gallon tank, one can comfortably keep up to 8-10 Platies:

  • Size and Temperament: Platies generally grow to about 2 inches and are peaceful, so they won’t intimidate or harass other tank mates.
  • Breeding Ease: Unlike the Oscar fish, Platies breed easily and can quickly populate a tank if males and females are present.
  • Water Conditions: Platies prefer slightly alkaline water but are adaptable to various water conditions.

2. Guppy

Guppies rank among the favorite choices for freshwater aquarium enthusiasts. In a 20-gallon environment, you can comfortably house 15-20 Guppies:

  • Upkeep: Guppies require minimal care, certainly less than the demanding Oscar fish.
  • Breeding Prolifically: Keep an eye on the male-to-female ratio since they breed rapidly.
  • Feeding Habits: Guppies enjoy a diverse diet, munching on both greens and proteins.

3. Neon Tetra

Neon Tetras are petite and exhibit radiant colors. A group of 15-20 fits perfectly in a 20-gallon tank:

  • Group Dynamics: They enjoy company, which magnifies their vibrancy and typical behaviors.
  • Aquatic Preferences: Slightly acidic water with consistent temperatures suits them best.
  • Friendly Disposition: Neon Tetras are peaceful, mingling well with various species.

4. Dwarf Gourami

Dwarf Gouramis are aesthetically pleasing and calm fish. 1-2 Dwarf Gouramis are ideal for a 20-gallon tank:

  • Physical Attributes: They attain a size of about 3.5 inches, fitting nicely in a 20-gallon environment.
  • Respiration Quirk: Unlike Oscars, they possess a unique organ to breathe straight from the air.
  • Color Palette: They come in shades like blue, red, and fiery orange.

5. Betta Fish

Bettas are colorful with distinct fin designs. A 20-gallon tank should host just 1 male Betta or a “sorority” of 5-6 females:

  • Solo Preference: Male Bettas are territorial; it’s best to give them their space.
  • Breeding Signs: In contrast to Oscars, male Bettas craft bubble nests when in the breeding mood.
  • Water Temperature: They favor warmer environments, typically between 76-80°F.

6. Zebra Danio

Zebra Danios love to move around. A group of 12-15 will find a 20-gallon tank spacious:

  • Constant Motion: These fish are lively, frequently zipping across the tank.
  • Cool Comfort: They’re more adaptable to cooler waters compared to Oscar fish.
  • Reproduction: Their egg-scattering is an intriguing sight.

7. Corydoras Catfish

Corydoras primarily stay at the tank’s bottom. For a 20-gallon setting, 6-8 Corydoras is a good number:

  • Cleanup Crew: They play janitor by eating up residual food and algae.
  • Community Seekers: Corydoras thrive in groups, offering protection and camaraderie, unlike the loner Oscars.
  • Variety: Numerous Corydoras species exist, each boasting its distinct designs and dimensions.


Here is a quick recap for those of you who are just skimming through:

  • A 20-gallon tank isn’t suitable for adult Oscars due to their growth, aggression, and space needs, emphasizing larger tanks.
  • Baby Oscars thrive in a 20-gallon tank for 6-8 months, but transitioning to more space is crucial.
  • A 55-gallon minimum tank size is essential for adult Oscars’ growth, behavior, and water quality.
  • Tank size considerations based on fish length, activity, and territoriality are vital for Oscar fish’s well-being.
  • Smaller fish like Platies, Guppies, Neon Tetras, Dwarf Gouramis, Betta Fish, Zebra Danios, and Corydoras Catfish suit 20-gallon tanks, each with unique traits.