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Best Tank Size for Oscar Fish (55, 65, 75, 90 or 125 Gallons?)

Oscar fish are a species of cichlid found mainly in the tropical waters of South America. They’re big, menacing-looking fish known for their black and orange markings reminiscent of a tiger. If you’re considering keeping this highly intelligent fish, you may be wondering what size tank is needed?

Oscars are big fish and grow to lengths of 12 inches or more. Therefore, a large tank (75 gallons) is required for just a single fish. They’re known to be highly aggressive and best kept alone or in mated pairs in an extra-large (125-gallons) species-only aquarium.

Now that you know you must keep Oscar fish in the biggest aquarium possible, let’s explore this topic further. Together we’ll learn how many you can have in a 55-, 65-, 75-, 90-, and 125-gallon tank, what happens when they’re housed in too-small of a tank, and what to do if you have too many in your tank.

So, if you’re ready to learn more about Oscar fish and what tank size is best for this big, energetic, and fierce species, then let’s begin!

How Much Aquarium Space Does an Oscar Need?

Oscars are large, highly active fish that need lots of space in an aquarium for swimming and foraging. Their energetic nature coupled with their heightened aggression requires they be kept on their own or in a mated pair in an-extra tank. A community tank environment isn’t recommended for this species.

How Much Space is Required for More than One Oscar Fish?

When stocking an aquarium, you must consider not only the size of the fish but their temperament as well. Oscars are very big and quite aggressive. Plenty of space is required to keep their often-hostile behavior at bay. They’re also very active and need room to swim freely and explore their surroundings.

How Many Oscars Can be Kept in a 55 Gallon Aquarium?

You can keep one juvenile Oscar in a medium-sized (55-gallon) tank. Oscars are large fish, by aquarium standards – reaching adult lengths of 12 inches or more! They’re ‘messy’ and eat a lot which can quickly result in an overload of detritus. Too small of a tank will make the water toxic and dangerous for them.

How Big Will an Oscar get in a 55 Gallon Tank?

An Oscar fish will grow to at least 12 inches in a 55-gallon tank. At that size, a 55-gallon tank is simply too small for this species. Anything less than a 75-gallon aquarium is not suitable for such a large fish. You can keep juveniles in a 55-gallon tank temporarily but be prepared to move them as they age.

How Many Oscars Can be Kept in a 65 Gallon Aquarium?

You can keep just one or two juvenile Oscars in a 65-gallon tank. The rule-of-thumb for stocking an aquarium is one inch of fish per gallon (or two) of water, though it’s not ideal for this very large, highly active species. This fish needs plenty of space to forage and explore in their aquatic surroundings.

How Many Oscars Can be Kept in a 75 Gallon Aquarium?

The minimum sized tank for a single adult Oscar is 75 gallons. Oscars are active fish and need lots of space to swim freely and explore their aquatic surroundings. In a too-small aquarium, this species will become sick, stressed, and/or lethargic – refusing to eat and laying motionless along the substrate.

How Many Oscars Can be Kept in a 90 Gallon Aquarium?

You can keep one full-grown Oscar fish in a 90-gallon tank. A pair of juveniles will do fine in a 90-gallon aquarium, but you must be willing to re-home them in a larger tank once they reach sexual maturity. This typically occurs at around 14 months of age when they reach a length of 8 to 10 inches.

lone Oscar fish

How Many Oscars Can be Kept in a 125 Gallon Aquarium?

You can keep a mated pair of adult Oscar fish in a 125-gallon tank. This species is very big by aquarium standards and needs an extra-large aquatic environment to thrive in captivity. If the tank is too small, Oscars will be become sick, stressed, and/or sluggish.

How Big Will an Oscar Get in a 125 Gallon Tank?

Though it’s been stated that aquarium fish will grow to fit their aquatic surroundings, this is a myth. Genetics is what dictates how big a fish gets, not environment. If a fish stays small in captivity, it’s the usually the result of other contributing factors such as lack of food, anxiety, or/or disease. An Oscar fish, if healthy and well cared for, will grow to 12 inches or more in 125-gallon tank.

What Tank Size is Best for 2 Oscars?

If you use the general rule for stocking an aquarium – one inch of fish per gallon (or two) of water – then you would think it’s okay to house two adult Oscars in a 50-gallon tank. However, for this large, highly active species, I’d recommended an inch of fish for every 4 gallons of water. Therefore, you should keep a mated pair in a 100-gallon tank minimum.

This type of fish is ‘messy’ and produces a lot of waste material. Oscars eat a lot and like to dig in the substrate – they’ll typically scoop it up in their mouths and spit it back out! This behaviour stirs up toxins and detritus, thus affecting the water parameters. In a too-small tank, the ammonia levels will rise too quickly and cause stress and/or illness for your fish.

What Do You Do If You Have Too Many Oscars in a Tank?

If you have too many Oscar fish in a tank, then you need to either get a larger aquarium to comfortably house them all or rehome some to a new tank. Oscars grow to be very large. They consume a lot of food and produce a lot of waste. In a too-small tank, the ammonia levels will reach dangerous levels.

If you can’t get a bigger tank right away, then you must ensure you have a high-powered electric filter installed to keep the water clean and safe. Regular weekly water changes of at least 15% are also a must to help keep their aquatic environment toxin-free.

Can You Keep Oscar Fish in a 10- or 20-Gallon Tank?

You cannot keep Oscar fish in a nano tank. A 10- or 20-gallon aquarium is far too small for such a large, active fish. If you attempt to keep one in this tiny of a tank, your Oscar won’t live long. It’ll succumb to the stress of being confined to an insufficient aquatic space and likely die.

A 10- or 20-gallon tank will also allow for toxins to build up in the water much quicker. Oscars are very sensitive to abrupt changes in water parameters and are highly susceptible to tropical fish diseases including ich, dropsy, and fin rot.

school of Oscar fish

Final Thoughts

To sum-up, Oscars are very big by aquarium standards. They can reach adult lengths of 12 inches or more in captivity. Hence, a large tank of 75 gallons is required for just a single Oscar fish – a mated pair needs an extra-large tank of 125-gallons.

I hope this article has been of help to you in determining what tank size is best for Oscar fish. Thanks for reading and good luck with your aquarist hobby!

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