Are Oscar Fish Aggressive? Or Just Misunderstood?

Nov 24, 2021

Are Oscar Fish Aggressive

Image credits: Ellenm1 (Creative Commons license)

Big, beautiful, curious, and intelligent – there are so many adjectives to describe an oscar fish, but there’s one word that stands out the most – aggressive. Are oscar fish aggressive? Or just misunderstood? 

Today’s blog revolves around oscar’s “anger problem.” First, I’ll tell you if they are really aggressive or not, and if yes, how to tackle it. 

Are Oscar Fish Aggressive?

Oscars are semi-aggressive fish. However, in the wrong environment, they can be really mean. They’re especially hostile towards smaller fish whom they view as a potential meal. But when kept with similarly-sized fish that can hold their own, an oscar’s anger can be managed. 

How Do Oscars Fight?

Oscars are intelligent fish, so they fight in interesting ways. For example, they will push around and jump at each other. They will also be involved in locking jaws!

The fighting fish will often swim through rings or down runways for an opportunity to take a hit at the rival – and funnily, they’ll sometimes even attack their own reflections. 

New studies have shown that how fish fight often boils down to the fish’s personality. 

Apparently, some attack ruthlessly and continuously, almost like boxers pummelling their opponent into surrender. Whereas others are more tactical, carefully studying the rival before deciding where and how to strike – just like martial artists. 

The tank’s hierarchy also comes into play during a fight. For example, Oscars will fight differently depending on whether their opponent is a junior or a senior. 

If it’s a senior, they do not tend to lash out haphazardly. 

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Why Are Oscar Fish Aggressive?

Aggression is an innate nature for most parts. They’re wired to be that way. However, often, environmental factors play a crucial role in triggering anger. For instance, an over-cramped tank means a territorial dispute, and insufficient food means fighting over resources. 

Off the top of my mind, these are the few reasons oscar fish resort to anger:

There’s Not Enough Space In The Tank

Oscars are big fish. They can reach well over 12 inches in captivity. Also, they grow fast. They grow at the rate of 1 inch per month in the first year. Astounding! 

So, even though the minimum recommended tank size is 55 gallons for a single oscar, I’d say a 75-gallon tank would be ideal. And for every additional oscar, add 30 gallons each. 

Fish get territorial when there’s a lack of space – it’s nothing surprising. Even tiny fish like guppies and tetras fight over territory. So you can only imagine what giants like oscars will do. 

Your Oscar Fish Is Stressed 

Oscars are hardy fish, but they’re also quite easily prone to stress. And when stressed, it’s natural for your oscar to act erratically and aggressively. 

Stress can be triggered by various factors. It can stem from a territorial dispute, lack of resources, mating competition, or simply a show of dominance. 

And fight breakouts aren’t the only side effects of stressed fish. 

When stressed, fish produce a hormone called cortisol in excess, which suppresses their immunity and inhibits the process of converting food into energy. 

Therefore, stress will negatively affect a fish’s health, compromise its immune, and expose it to secondary illnesses. 

Your Oscar Wants To Show Dominance 

Oscars are intelligent fish. As absurd as it sounds, they also have a complex social hierarchy and a pecking order in place. Often, fish just need to learn and accept their position in the pecking order for things to settle down. 

The pecking order is often determined through aggression and direct competition. The dominant ones will bully the weaker ones. Older fish will rule over young ones, and a fish living in a specific locale for a longer time will outrank newcomers. 

Basically, dominance is determined by size, age, and experience. 

And the pecking order is subject to change as various individuals grow and mature. So even tanks that have been peaceful for a long duration will go through periods of upheaval from time to time. 

There’s Not Enough Food In The Tank 

Fighting over food resources is, I think, the number one reason behind an oscar’s aggression. If food is sparse, they will fight and drive off any fish they perceive to be a threat to their meal. 

I won’t blame them! Oscars are big fish with a pretty big appetite. 

That’s why, when feeding, always make sure to spread out the food evenly throughout the tank. Offering a variety of food simultaneously can also help. 

We are often fed with copious amounts of information on how we shouldn’t overfeed your fish. Yes, it’s definitely true. 

However, there’s an equal or even greater chance of underfeeding, too. 

And this won’t just make your fish fight over resources but quite seriously impact their health as well. 

Malnourishment is just as big a problem as obesity, if not more. 

For instance, if your fish lacks vitamin A in its diet, it’ll be susceptible to conditions like fin-base hemorrhage and eye problems. 

Likewise, lack of vitamin D causes poor growth, while vitamin E deficiency results in muscular wasting. 

The Water Parameters Are Not Right

If the water parameters are not suitable, it will stress your fish and negatively impact its health – forcing it to resort to displaying erratic behaviors like fighting. 

Like all fish, oscars get profusely stressed if the water parameters are incorrect. And we can’t really blame a stressed fish for losing its temper. 

As you already know, oscars grow pretty big. So naturally, they produce an impressive amount of bioload every day. Therefore, you need to create a robust filtration mechanism and a routine cleaning regime to stay on top of the water quality. 

When exposed to the wrong environment, a fish’s health will deteriorate. And a sick fish will act irritably. So even small things, which oscar would have ignored otherwise, can be the reasons behind the confrontation. 

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How To Stop Oscar Fish Aggression?

Oscars are hardwired to be somewhat aggressive. They’re born this way. However, you can still help manage and lessen their aggression by providing a big tank, feeding the right way, creating a stress-free environment, and adding compatible tank mates. 

Provide A Big Tank

As stated above, the ideal tank size for a single oscar is 75 gallons, and the absolute minimum tank size is 55 gallons. You should add 30 gallons extra for each additional fish. 

If the tank is big, there will be ample space for everyone to claim their tiny territories. This way, they won’t bump into each other often and fret over the ideal spot for breeding. 

Besides controlling stress, there are other equally important benefits of a big tank. First, your fish will get plenty of exercise and thus grow to their full-size potential. Their growth will not be stunted. 

Next, bigger tanks are more stable and more forgiving of fluctuations in water parameters. The smaller the surface area, the more temperamental the tank will be. 

And lastly, the hormones and pheromones that fish secrete in the water to inhibit the growth of other fish will get diluted and washed away easily in a bigger tank! 

Feed The Right Way 

Oscars are avaricious eaters. They are greedy. Therefore, they will often beg for food and continue eating even when they’re full. From what my vet told me, feeding 3-4 small meals a day instead of 1 or 2 big meals can help curb aggression to some extent. 

Each time, give them an amount they can finish within 30-40 seconds. 

Half a minute can seem like a very short window, but remember, oscars are voracious eaters. They can gobble down a sizable amount even in short periods. 

However, if you have young oscar fry, you will need to feed them around 5-6 times a day. They mustn’t miss the meal as they’re very much susceptible to starvation. 

Also, make a point to disperse the food equally in all areas instead of pouring everything in one spot. 

Although omnivores, oscars lean towards meaty diets whenever possible. In the wild, they devour small fish, crustaceans, worms, and insects. 

In the tank, you can give them quality pellets or flakes as part of their staple diet. Then you can occasionally supplement with options like frozen mealworms, brine shrimp, and live fish. 

Avoid giving food like beefheart as it leads to fat deposition in the liver, tubifex as it’s often infested with parasites, and bloodworms since they’re linked with bloating! 

Create A Stress-Free Environment 

Stress is among the top reasons behind an oscar’s aggression. So you cannot blame a stressed fish for causing a scene, right?

Now, the tricky part is deciding what is, in fact, causing the fish to stress and resort to violence. The reasons can be many. It can be an overpopulated tank, a polluted tank, a lack of resources, two alpha males, and so on. 

Some palpable signs of stress in fish are: 

  • Loss of appetite 
  • Frequent hiding behavior 
  • Darting around the aquarium 
  • Frantic swimming 
  • Laying at the bottom 
  • Panting

And the worst thing about stress is not unsolicited fight breakouts – it’s potential diseases. Stress will compromise a fish’s immunity and expose it to a plethora of possible secondary diseases. 

And if an oscar isn’t feeling well, it will not act well. 

So, if your fish shows any of the above signs besides aggression, try to experiment with some subtle changes until you find out what’s causing it to stress and act a certain way. 

Maintain Water Parameters 

How would you feel if you were kept in a filthy, polluted room? Unfortunately, that’s precisely how oscars feel when the tank’s water parameters are just not right. 

If the environment is unsuitable, it will cause both mental and physical stress to your oscar fish, who will then naturally lash out at each other to express the frustration. 

So, even though wrong water parameters may seem like a far-fetched reason for an oscar’s anger problem, it isn’t so. 

Always make a point to test the water quality if your oscar is showing signs of irritability or aggression. In most cases, there’s always something wrong. 

Here’s what the parameters should look like for an oscar fish:

Temperature74-81 °F
pH6.8-7.2
General Hardness70-140 ppm
Carbonate Hardness80-100 ppm
Ammonia 0-0.25 ppm
Nitrite0 ppm
NitrateBelow 20 ppm

Remember, oscars come from the soft and acidic waters of the Amazon river. If the water’s too alkaline, it will dissolve your fish’s protective mucus layer and make it vulnerable to secondary infections. 

The rule of thumb is to change 25-30% water every week, but it’s not set in stone. It boils down to how big your tank is and your stocking number.

Also, don’t go overboard with cleanliness. Changing water and cleaning more often than you need to will destroy the good bacteria colony, which in turn, comes with its own set of problems. 

Add Hiding Places 

It doesn’t matter how big an oscar’s tank is if there aren’t hiding places where the fish can quietly retreat. Although incredibly ballsy, oscars also need to retire to a quiet place and recharge their batteries often. 

Therefore, add plenty of decors like plants, rocks, and caves to the tank. Place them strategically so they don’t interfere with the swimming area. 

Adding piles of rocks strategically to break the line of sight is an ingenious way to break the line of sight, which helps to control aggression. 

Choose The Right Tankmates 

Oscars are big and feisty. They also have a strong predatory instinct. Therefore, they should not be kept with just about any fish.

Oscars have a reputation for trying to devour anything they can fit inside their mouths. So you have to choose their tank mates ideally. 

In order to qualify as an oscar’s tankmate, a fish must be similarly sized and be able to hold its own against the oscar’s fury. 

When kept with such tankmates, oscar’s aggression becomes subdued, and they become increasingly tolerant of other presences in the tank. 

Some assertive fish that can survive and thrive in the same tank as oscars are convict cichlids, firemouth cichlids, jack dempseys, green terror cichlids, and severum cichlids. 

And that takes us to our next part, where we’ll quickly look into the most compatible tank mates for oscars! 

5 Best Tankmates For Oscar Fish

  • Convict cichlids
  • Jack dempseys 
  • Green terror cichlids 
  • Firemouth cichlids
  • Severum cichlids 

Convict Cichlids 

convict cichlid
Credits: David Ellis (Creative Commons license)

  • Compatibility Rank: 10/10
  • Care Level: Easy
  • Size: 6 Inches
  • Temperature: 74-82°F

Convict cichlids only grow around 6 inches long, but boy, are they feisty! Don’t go by the size. These fish are territorial and aggressive – two things we need in an oscar’s tank mate. 

Convict cichlid’s tough and hardy nature means they can easily hold their own if the oscar ever gets mean. However, remember, the convict cichlid should be fully grown before adding it to an oscar’s tank. 

A 2-inch-long juvenile convict can stand up to an oscar, right?

Jack Dempseys 

Electric blue jack dempsey cichlid
Credits: PINKE (Creative Commons license)

  • Compatibility Rank: 7/10
  • Care Level: Easy
  • Size: 10 Inches
  • Temperature: 78-82°F

These cichlids are named after legendary American boxer Jack Dempsey, and they sure do live up to the name. These fish grow up to 10 inches long – thus, they are perfectly compatible to be kept with an oscar. 

Since jack dempseys grow almost as big as oscars and match their aggression level, they’re often the top choice for an oscar’s tankmate. 

Green Terror Cichlids 

Green terror cichlid
Credits: Fcbaum (Creative Commons license)

  • Compatibility Rank: 7/10
  • Care Level: Moderate
  • Size: 8 Inches
  • Temperature: 72-80°F

Green terror cichlids aren’t exactly green, but they’re just as fierce as their name suggests. Also, they grow around 12 inches long, just as big as an oscar. 

When searching for suitable tankmates for the oscar, we often need to be worried about oscar being the bully. However, with green terrors, it’s exactly the opposite!

In my opinion, if there’s one fish that can go head to head with an oscar, it’s a green terror cichlid. 

Firemouth Cichlids 

Credits: 5snake5 (Creative Commons license)

  • Compatibility Rank: 8/10
  • Care Level: Easy
  • Size: 6 Inches
  • Temperature: 75-84°F

Firemouth cichlids are closely related to convict cichlids, and that’s where their aggression comes from. Firemouth cichlids grow only about 6 inches long but are confident and strong-willed enough to thrive in an oscar’s company. 

Another interesting thing about firemouths is that they’re great escape artists. If there ever comes a situation when they can’t take on an oscar head to head, they’ll cunningly dart away in an instant. So, make sure to add plenty of hideouts in the tank. 

Severum Cichlids 

Severum cichlid fighting
Credits: Byron Chin (Creative Commons license)

  • Compatibility Rank: 8/10
  • Care Level: Moderate
  • Size: 8 Inches
  • Temperature: 73-77°F

Severum cichlids are the most peaceful and subdued cichlids on this list today. However, as docile as they are, they can still stand up against an oscars if needed. 

These fish grow over 8 inches long. And although that’s quite considerably less than an oscar’s size, these fish will hold their ground when needed. 

But the best thing about severums is that they’ll keep to themselves. So they’ll seldom get in an oscar’s way. 

Can Toys Help Control Oscar’s Aggression? 

Yes, definitely. Oscars are highly intelligent fish. Therefore, they’ll greatly benefit from a few toys in the tank. This will help them keep occupied and happy. Therefore, they won’t end up lashing out at tank mates as frequently as they do. 

Scientific breakthroughs every day prove that fish minds aren’t as primitive as we previously used to think. Instead, they’re complex, developed, but most importantly underrated. 

I think it’s especially true for an oscar. After all, they’re known to greet their owners by shaking their heads and tails, begging for food, and eating right from the hand. 

Don’t miss this adorable video of an oscar fish playing with a tiny soccer ball and eating from hands! 

3 Types Of Toys For Your Oscar Fish

Credits: Jennifer Rouse (Creative Commons license)

  • Ping pong balls 
  • Water Wheels 
  • Moss balls

Ping Pong Balls 

As shown in the video above, your oscars will have a ball playing with a ball. Sorry for the bad pun. With little patience, you can even teach your fish to knock the balls into little nets. 

Just make sure balls aren’t too heavy to inflict injuries or are dyed with paints that can be toxic for the fish. 

Water Wheels 

I am adding water wheels to the list because one hobbyist on a forum mentioned it. Reportedly, his fish learned to turn the wheels by adding stones on it – how exciting! 

If you doubt your oscar will be able to maneuver the water wheel manually, you can purchase battery-driven wheels at any pet shop. 

Moss Balls 

Just like the ping pong balls, moss balls can make great additions to your oscar’s tank. The fish will love to push it around and play with it. 

Moss balls are incredibly hardy, too. So, you don’t have to worry about your oscar destroying them. But if you’re still concerned, you can use an artificial moss ball that can also actually trap phosphates and nitrates. 

Frequently Asked Questions

Credits: Allan Reyes (Creative Commons license)

Why Are My Oscars Fighting All Of A Sudden?

Oscars fighting is not an uncommon sight – it’s expected behavior. However, if you see your fish fighting out of the blue, most probably there’s something wrong with the environment. 

Maybe the tank has become overcrowded, and they’re fighting over territory. Maybe the polluted environment is causing them to lash out at each other. And lastly, it could be for establishing a new pecking order in the tank. 

Are Parrot Fish More Aggressive Than Oscars?

No, parrot fish don’t even come close. At worst, they are semi-aggressive. And they’re often considered peaceful compared to aggressive species like oscars, green terrors, and convicts. 

On top of that, parrot fish have deformed mouths that don’t function normally. So, even if they get angry and try to harm your oscar, their dysfunctioning mouths will not allow it. 

Final Words: Are Oscar Fish Aggressive?

Yes, oscar fish are aggressive. They’re fierce, dominant, and hostile. But they’re also largely misunderstood. 

When kept in the right environment with suitable tankmates, their anger can be significantly toned down. Therefore, it’s vital to pay keen attention to factors like offering ample space and hideouts, feeding adequately, and pairing only with fish that can match their size and aggression. 

Since oscars are highly intelligent fish, they will also benefit from adding a few toys like a ping pong ball or a moss ball. These will keep the fish occupied throughout the day. 

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rohit gurung author at urbanfishkeeping

About Rohit Gurung

My never-ending love and fascination with Aquascaping started when I received a red-eared turtle for my 10th birthday.

Apart from researching and writing, I spend hours gazing at my 3 turtles. And yeah, I bask alongside them too.