Oscar Fish Tank Size (The Wrong Size Can Kill Your Fish)

Oscar Fish Tank Size

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Oscar fish are known for their large size, beautiful color, and loyalty to their owners. 

They are among the few rare fish that acknowledge and remember the hand that feeds them. In return, all the Oscar fish asks for is regular feeding, amiable tank mates, and a well-maintained tank.

Since this freshwater species can grow up to 18 inches in length, it’s crucial to provide them a large enough space to swim freely. Needless to say, responsible first-time owners often wonder what the ideal Oscar fish tank size is. 

Here are a few things to remember before buying an aquarium for your beloved Oscar fish.

The (False) Recommended Oscar Fish Tank Size

Don’t believe everything you read on the internet, especially when it comes to pet care. For example, a few online sources claim that even 30-gallon tanks are suitable for Oscar fish. However, any experienced aquarist will agree that anything around 75 gallons is not recommended.

What’s The Minimum Tank Size For Oscar?

Whether you are bringing home a Tiger Oscar, an Albino Oscar, or any other variation of the Oscar fish, all of them have the potential to grow to a large size. That being said, aim for a 75-gallon tank as the minimum tank size for a single Oscar. 

What Size Tank Do I Need For 2 Oscars?

For most aquarists with a single Oscar fish, a 75 to 90-gallon tank is the norm. Although some aquarists have had luck keeping two Oscar fishes in a 90 to 100-gallon tank, this poses the risk of diseases, such as Hole in Head (HiH) and Lateral Line Erosion.

After all, a large size tank is not only meant to provide space to move around. In fact, the voracious appetite of Oscars leads to an exponential buildup of fish waste. This bioload further produces ammonia, one of the most common culprits of poor water quality in aquariums.

Moreover, leftover scraps of fish food break down into nitrate compounds that can prove to be toxic for any fish. As a rule of thumb, ensure that nitrate levels stay below 20-30PPM at all times.

With all that said, a 75 to 90-gallon tank can work perfectly fine during the grow-out period of two baby Oscar fish. However, once these have matured and reached 10-18 inches in length, you will need to place them in a 150 to 180-gallon tank. 

In fact, you can start your own cichlid fish tank by introducing a few other cichlids to the tank once the pair of Oscar fish is well-settled. 

Will An Oscar Outgrow Its Tank?

Based on anecdotal evidence, it has been observed that an Oscar fish witnesses stunted growth when put in a tank that is too cramped. Although it may seem like the fish is adapting to its surroundings, this is actually a sign that there are problems with its environment. Moreover, some fish do grow to their potential size, albeit at a much slower pace.

Nevertheless, it is essential to remember that the discrepancy in their growth rate is not a result of the tank size itself but because of poor water quality. In fact, choosing a small tank for a large fish, such as an Oscar, is akin to making the fish swim in its own waste.

Even with a sound filtration system in place, nitrate, nitrite, and ammonia in a small tank reach harmful levels. This not only leads to stunted growth but also drastically reduces the lifespan of the fish.

At the same time, we cannot completely disregard the fact that a smaller size tank perhaps leads to tiny Oscars due to insufficient space for natural movement. Either way, a bigger tank for an Oscar will lead to more beautiful and active fish that are a delight to have around.

Can A Tank Be Too Big For Oscar Fish?

When it comes to picking a fish tank size for an Oscar fish – the bigger, the better. In fact, the Oscar fish is native to the waters of the Amazon river, which is considered the most extensive river globally, in terms of the volume of its flow.

Needless to say, no tank in the world can be too big for the Oscar fish, which thrives in a vast expanse of water when in the wild.

However, it is essential to note that the Amazon river lies in the tropics of South America, and the warm waters are well-suited for the fish. Therefore, most aquariums will require a heater to provide a comfortable water temperature of about 70-80 degrees Fahrenheit.

Can An Oscar Live In A 10, 20, or 30 Gallon Tank?

In the wild, Oscar fishes grow up to a length of 18 inches, and in captivity, they may peak at about 10 to 14 inches. Needless to say, a growing Oscar will require much more than a 10, 20, or 30-gallon tank for a healthy and long life.

The ideal fish tank would be 4ft. long and 2ft. wide, and can hold at least 75 gallons of water. Be advised that this is a minimum requirement for a single fish. If you are planning to raise a pair of Oscars, make sure to double the volume of water and the size of the tank for optimal growth and health


While Oscars are an amiable fish, they are undoubtedly a high-maintenance species that require at least a 75-gallon tank per fish. Even buying the right fish tank size is not enough – proper care and maintenance of the tank itself are crucial. 

This is in addition to regular water changes and fitting the aquarium with necessary devices. Filtration systems, in particular, are non-negotiable when dealing with Oscars, which produce a lot of fish waste. 

Moreover, a large tank that holds a greater volume of water facilitates dilution of the fish waste and aids the filtration systems. To sum it up, an ideal fish tank size leads to better water quality and ample space for movement. This, in turn, provides Oscar fish a long and happy life.

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Jeff Colt

Jeff Colt

Hello, I'm Jeff- an aquarium enthusiast with over 25 years of experience caring for a wide array of tropical fish, including koi, goldfish bettas, cichlids and more! For me: Aquariums are like jello - there's always room for more!

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