How Big Do Oscars Get? (In Captivity & Wild)

Oscars are large and messy, and so much more. 

Many aquarists love to have them around the house because of their intelligence and interesting personalities. In fact, they are lovingly called “river dogs” because of their loyalty and ability to recognize their owners. 

That being said, Oscars have big personalities, which means they can be moody, aggressive, calm, happy – all on the same day. To keep aggressive behavior at bay, it’s crucial to put Oscars in a tank that simulates their natural habitat and is large enough for them. 

And so, the question arises, “How big do Oscars get?”

How Big Do Oscar Fish Get In Captivity?

In captivity, full grown Oscars can measure up to a maximum of 12 inches – a healthy 10 inches being the norm. However, their healthy growth depends on various factors, such as tank size and nutrition. In fact, keeping an Oscar alive requires suitable environmental conditions. If done right, this species of fish can live to be as old as 20 years old. 

How Big Do Oscars Get In The Wild?

Most wild Oscars will be about 18 inches in length and weigh around 3lbs in the wild. when they are fully mature. However, these numbers may vary according to the environmental conditions and specific breed. In fact, the famous Red Tiger Oscar cichlids, most notable for their large dark bodies and golden spots, are mostly found thriving in the wild.

How Fast Do Oscar Fish Grow In Captivity?

Some aquarists claim that their fish went from 4 inches to 10 inches in a matter of four months, whereas others recorded a growth of 1.5 inches in the same period. Yet another pet owner noticed that their Oscar fish was fully mature within two years, which is when the fish revealed its gender. Needless to say, there is no fixed growth rate of Oscars.

6 Factors that Determine How Big Oscar Can Get

When the maximum size was measured at maturity, most of the Oscar fish in captivity were significantly smaller than those in the wild. Needless to say, the Oscar fish thrives most in its natural surroundings in the slow-moving waters of the Amazon basin. 

At the same time, they can live adequately healthy lives in a desirable environment even in captivity. In the end, the growth of an Oscar fish depends on the size of the tank, filtration, and food provided. 

Tank Size 

Oscars are generally not considered to be beginner fish because of the large aquarium that they require. While some online sources recommend a 40-gallon tank for a single Oscar fish, that number should, in fact, be doubled. As a rule of thumb, 75 gallons should be comfortable for a single Oscar fish, whereas a minimum of 100 gallons should be enough for a pair.


When it comes to large fish with a big appetite, don’t be surprised if they leave a large bioload behind. Therefore, it is highly crucial to have a proper filtration system that can keep the aquarium water waste-free for the most part.

A combination of a canister filter and a HOB (hang-on-back) filter will provide sufficient filtration, as long as the aquarium is large enough to dilute the waste. Needless to say, a small tank is a recipe for murky waters, which can cause illness or prove to be fatal to the Oscar fish


In the wild, Oscars are predatory fish that primarily feed on insects and other small fish. Live food, such as worms, crayfish, feeder fish, and insects, is suitable for those fish in captivity. Alternatively, Oscars devour thawed meat-based frozen fish foods with much delight.

Water Condition

The Oscar fish requires the same tank water conditions as most freshwater species. So, begin preparing the aquarium by placing an under-gravel filter and 3 to 5 inches of substrate at the bottom.

After this, apply a conditioner or let tap water stand for at least 24 hours before pouring it into the tank. Since Oscars are highly sensitive to temperature, use a heater to replicate the warm water of the tropics. Ideally, the water temperature should be 77°F, but anywhere between 74 and 81°F should be comfortable enough.

Tank Mates

Placing a smaller fish with Oscars is a bad idea since they will gobble up any that fits into their mouth. These predatory fish are notorious for being aggressive not only to smaller fish, but also to their own kind. 

In fact, any timid fish will get stressed out by the aggressive antics of this fish. On the other hand, confident fish, including types of catfish and cichlids, will likely get along with Oscars.

You might be interested: 7 Best Oscar Fish Tank Mates (Compatible And Safe)

Tank Maintenance

Given their large size and food intake, Oscars are likely to produce larger amounts of ammonia, which is a primary component of fish waste. While chemical filtration is necessary to remove toxins and keep the water clean, regular water changes are also absolutely required.

Furthermore, if you plan to use a submersible heater, ensure that it is shatter-proof as Oscars are destructive. What’s more, they delight in jumping out of their tanks, so remember to leave the aquarium shut at all times.


How Fast Does A Baby Oscar Grow?

Some baby Oscars can grow from half-an-inch to one inch a month until they are 10-18 inches. On the other hand, some may never even grow at all. In the end, it’s their living conditions and nutrition that will decide the growth rate.

Do Oscars Grow To The Size Of The tank?

If the tank’s size is appropriate and is filled with at least 70-75 gallons of water, it is unlikely that the Oscar will grow to that size. At the most, these species of fish peak at 18 inches in length.


To sum it up, Oscars are relatively high-maintenance fish with a voracious appetite and an aggressive streak. However, with proper care, they can grow to be large 10 to 18-inch fish that are magnificent to look at. 

In fact, variations and fancy Oscars, such as the albino Oscar and Lemon Oscars, are some of the rarest and most expensive fish in the sea and aquariums. 

That being said, taking care of one may prove to be challenging if you wish to see it grow to its maximum potential. But, at the same time, an Oscar’s beauty, mischief, intelligence, and loyalty are what make it so valuable. 

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