I hope you love the products I recommend! By the way, any links on this page that lead to products on Amazon are affiliate links and I earn a commission if you make a purchase. Thanks in advance for your support!

With that out of the way.

In this article, we explore aggression, hostile behavior common in fish, which you should be aware of, and we profile 6 of the most aggressive freshwater fish!

Personally, I’ve never kept aggressive freshwater fish in any of my aquariums.

Okay, that’s a bit of a lie;

I’ve kept Bettas and many different kinds of cichlids. But, I’ve never really kept the kind of fish that might make you afraid to stick your fingers in the water.

So I decided to reach out to my friends Alex Henry and Hugo Turner over at needyfish.com and asked them what they thought about keeping aggressive fish in a freshwater aquarium

Here’s what they had to say.

What Is The Definition Of Aggression?

Let’s step away from the tank for a second and explore what the word aggression means.

The dictionary states, “feelings of anger or antipathy resulting in hostile or violent behavior; readiness to attack or confront.”

Aquarium Maintenance Checklist [Free Guide]

On the topic of the most aggressive fish, Hollywood has done a great job of bringing the Piranha front of mind.

For those James Bond lovers, who can forget that famous scene from “You Only Live Twice.”

So Is Aggression A Bad Thing To Have In Your Tank?

To answer this question, first, you need to understand that there is a difference between an aggressive fish and a predatory fish.

Aggressive fish is that playground bully; they’re territorial, they fight over food, and will fight over mating.

In contrast, predatory fish may not show any of the above attributes but will feed on live fish.

Just because a fish is aggressive doesn’t automatically make it predatory.

Many aggressive fish are incredibly rich in color, making them a popular choice for aquarists.

When kept with suitable fish, or shall we say tank mates, they can be happily part of a thriving aquarium.

If you want to see predatory behavior in its raw form within a tank, please watch this, where you witness Piranhas feeding, viewer discretion encouraged.

Why Are Some Freshwater Fish Aggressive?

Well, funnily enough, a fish will fight over pretty much the same things we humans do.

Territory, friends, and romantic partners.

Yes, some fish are more aggressive than others. But that’s not to say certain fish don’t ever get aggressive.

Aggression can often be seen as a warning sign when they’re sick, or vice versa, with healthy fish seeing the open goal to pick on the weaker fish.

It’s helpful to observe the aggressive fish see if there’s an underlying cause.

Their territory is the most common reason.

Why?

Well, the spot they’re guarding is their claimed home. It’s the place they can breed, rest, and hide.

How would you like it if you had some randomer constantly trying to move into your house?

You see, these guys are a lot more like us than you think. But instead of a house, it’s a small rock or piece of wood.

More often than not, a fish is most likely going to get aggressive towards another fish of the same sex and breed. This is because they are most likely to be seen as a rival.

The primary reason for holding ground is to have a safe place to keep eggs or fry (baby fish).

So if you have some bloke knocking on your door who looks the same as you with the intentions of moving in, it’s only natural to feel a little aggression.

What Does Aggression Look Like?

Unfortunately, more often than not, you won’t necessarily catch the fish in the act of being aggressive.

This is not because they have some sort of telepathic sense that they know when you are in the room looking at them. This is down to the fact that who has 12 hours a day to be continually looking into the tank.

The important things to look out for are the signs that there has been some aggression.

This could be split fins, scratches, scrapes, changes in territory. Look out for the apparent wounds you’d expect from a scrap.

How To Reduce Aggression?

Rearrange your aquarium – By rearranging your tank’s layout, it will help level out the playing field for all the tank’s inhabitants, from the hang on the back filter to the driftwood. Move it all around to make it all appear as different as possible.

Add new hiding spots – Some fish are complete pushovers and will not stick up for themselves. By adding more hiding spaces for those fish will give them a place of refuge against the aggressors. Tall plants with a thick forest of stems are a good example.

Remove the bully – A “net breeder” would be a good place to start. By isolating the Bully for a week or two, allowing it to see and be around the other fish can help settle it once released again.

Get a bigger tank – This may seem obvious, but giving the fish more space to live in can work wonders. As most of the time, the aggressive fish get aggressive because the other fish keeps crossing them. So spacing them out is an excellent way to curtail the fighting.

Use a net to break up the fight – If you catch the fish fighting, using a net to break them up is an ideal temporary solution.

Tank Conditions – Make sure you have appropriate tank conditions, such as the correct pH level. Fish get stressed and, therefore, lean towards aggression if their environment starts to act against them. Changing the water consistently can help maintain your ideal environment.

6 Renowned Aggressive Freshwater Fish

1. Flowerhorn Cichlid

Flowerhorn Cidhlid

  • Origin: Central America and Malaysia
  • Lifespan: 8 -12 years
  • Care Level: Beginner
  • Size (Maximum): 10-12 inches
  • Water conditions: pH 6 – 8.5
  • Ideal tank: 75 gallons + minimum

Known for their distinct shaped heads, Flowehorn Chilchilds first appeared in Malaysian markets in the 1990s.

They are human-made hybrids and only exist in the wild because of their release. Their closest relatives are the South African Cichlids.

They have a reputation for being aggressive, but with careful planning, they can easily be managed.

Having them in pairs is recommended since they like to swim together, helping lower their stress and aggression levels.

2. Red-Bellied Piranha

Piranha

  • Origin: South America
  • Lifespan: 10 -15 years
  • Care Level: Intermediate
  • Size: 10 – 12 inches
  • Water conditions: pH 5.5 – 8.0.
  • Ideal tank: 40 gallons is fine for a single fish, but a group will need an aquarium of 100 gallons or more.

I’m sure the Piranha needs no introduction. There are estimated to be around 20-50 different species of Piranha. However, the most popular one for aquarists is the Red Bellied Piranha.

Despite what many people are led to believe, piranhas are omnivores, meaning they eat anything.

Piranhas also produce a lot of waste since they are very messy feeders. Hence, having a power filter to handle this is imperative to keep them alive and well.

3. Wolf Cichlid

  • Origin: South America (Costa Rica, Honduras, and Nicaragua)
  • Lifespan: 25 – 30 years
  • Care Level: High
  • Size: Maximum size of 35.5 inches
  • Water conditions: pH: 7.0 – 8.0
  • Ideal tank: Large – professional home installment (300 + gallons minimum) or public aquarium

With the official name of “Parachromis dovii,” the Wolf Cichlid is the largest of the Cichlid family.

It is a very serious predator and will consume anything that it can get into its mouth; therefore, please consider carefully what you intend to put in the tank with it (they have been known to co-exist safely with large armored catfish).

The Wolf fish is a very messy eaters, so your home tank substrate should consist of sand, making it much easier for you to clean.

4. Betta Fish/Siamese Fighting Fish

Betta Fish Flare

  • Origin: Asia (countries include Thailand, Cambodia, Vietnam)
  • Lifespan: 3 – 5 years
  • Care Level: Beginner
  • Size: 3.5 inches
  • Water conditions: pH 6.8 – 7.5
  • Ideal tank: Betta’s can survive in small spaces; bigger is generally better for your fish’s sanity. However, we wouldn’t advise you going less than 2.5 gallons.

Betta’s are one of the most popular freshwater fish and widely sought after for their beauty, ability to live in small spaces, and character, and I mean aggression when I say the character. This is where the siamese fighting fish gets its name from.

Betta’s are carnivores mostly eating insects and their larvae in the wild.

Male Betta’s love fighting, especially if it’s against other males; what you tend to find is the females fleeing into cover to avoid any entanglement.

Male Betta’s can co-exist with other fish species within a tank, in particular guppies.

5. Jaguar Cichlid

Jaguar Cichlid

  • Origin: South America (Costa Rica, Honduras)
  • Lifespan: 13 -15 years
  • Care Level: Medium – High
  • Size: 12 – 16 Inches (in the wild, they can grow to 2 feet plus)
  • Water conditions: 7.0 – 8.7
  • Ideal tank: 100 gallons + as a minimum size (even if you just have juveniles)

What is it about Cichlid’s that make them so aggressive!! Jaguar’s are no different and are extremely territorial fish, looking to pick a fight with anyone and anything, even if it’s bigger than them.

They are carnivores and a predator, so again if you intend to add small fish into your tank, be aware that this may be seen as feeding time for the Jaguar.

These are very active fish and can live with other Jaguar’s, especially in pairs; bonded pairs can successfully coexist.

6. Arowana

Arowana

  • Origin: Southeast Asia, South America, and Australia
  • Lifespan: 10 – 15 years
  • Care Level: Advanced
  • Size: up to 3 feet
  • Water conditions: pH 6.0 – 7.0
  • Ideal tank: 250+ gallons minimum

Arowana’s are extremely aggressive carnivorous fish, highly territorial, and will attack anything within your tank; they have the ability in the wild to jump several feet out of the water to catch small birds and insects.

They can grow to several feet, and as they continue to grow, the aggression continues to increase.

They are in high demand, and therefore, they are costly, with a high level of care and expertise necessary, therefore not great for a beginner fish enthusiast.

Final Words

Overall, you can have a lot of fun with aggressive fish; however, on aggregate, the level of care necessary to house and maintain aggressive fish is greater than their more docile companions.

Yes, they can be beautiful and add real character to your tank; however, please make sure you know how to care for them, and you understand what, if any, tank mates are suitable.

Tank On & Watch Your Fingers!