Rainbow shark and two GloFish Tetras

How to Stop Fish Bullying Other Fish: Working Tactics

Fish are beautiful and calming creatures.

Until they aren’t.

Some species are naturally aggressive, others become pesky bullies under certain circumstances. Bettas are among the most popular beginner fish with strong territorial tendencies. Many cichlids don’t shy away from conflict either.

But while bettas and cichlids are known quantities, many popular community species can create unexpected issues for their tankmates.

Angelfish, gouramis, rainbow sharks, some plecos, and occasionally danios can be aggressive. Their aggression can stem from environmental factors like limited space, social factors (insufficient members of a shoal or school), character, or a few more that we discuss below.

The most common ways to stop fish bullying other fish are:

  • Add other members of the same species
  • Rearrange the tank to reduce territorial behavior
  • Introduce new fish in the night
  • Reduce the water temperature by a couple of degrees (within tolerable limits)
  • Put the aggressor in a time-out box for a few days
  • Take it out of the tank for a few days

But before we examine each way to reduce bullying, we should understand the underlying reasons for aggressive behavior. Knowing the cause of fish bullying will help us design the best prevention approach.

The Main Reasons Why a Fish Is Bullying Another Fish

Some species simply are aggressive.

They are territorial, don’t tolerate other species, or like to eat smaller fish.

Others step on the path to bullying due to environmental factors.

Aggressive Fish Species

Cichlids are probably the most prominent family in the hobby with aggressive tendencies, but they aren’t exactly beginner-friendly.

Among the entry-level fish, bettas are the most aggressive. Many people start with a single betta, which, given enough space, is a good starter fish. They are sturdy, beautiful, and don’t need anything special to thrive.

The problem begins if you try to introduce other fish, betta, or another species. Bettas can be socialized from a very early age, but if they are used to living alone, they will harass others.

In turn, zebra danios can start nipping fins. A slow-swimming betta can become their target and so can a male guppy with a large tail.

Angelfish also can be territorial. They are part of the cichlid family and sometimes exhibit aggressive tendencies, particularly toward other angelfish. Small fish also can be targeted.

Gouramis, especially males, can start fighting for no good reason. They may be best friends, too, but if they don’t get along the weaker one will suffer mightily.

Tiger barbs also can harass slower, more passive fish.

Rainbow sharks are among my favorite species, but they can be very territorial, especially if you keep more than one in a tank under 30 gallons or 120 liters. And that’s when they are small. When they grow up, a much larger aquarium will be necessary to peacefully house two or more rainbow sharks.

It is important to mention that fish temparament varies within the same species. Most gouramis are peaceful and most danios are happy hyperactive swimmers. Rainbow sharks also can be well-adapted to a community tank.

But some are simply assholes.

Fish Bullying Due to Poor Conditions

As discussed below, many fish can be stopped from bullying others with the appropriate setup. That’s particularly true in the case of the below species. They aren’t inherently aggressive but may start harassing others if their living conditions are poor.

Plecos (plecostomus) are among the most popular cleaning-crew species in the hobby. They come in different colors and sizes, and, in general, are very amicable members of a community tank. But if their food is insufficient, they may start picking up on each other. As usual, the weaker fish will suffer.

The most common reason for peaceful fish to harass others is not food, though. In fact, most budding fishkeepers tend to overfeed their aquatic pets, which is a problem in itself.

Insufficient space is the main reason why a fish that is generally amicable can start bullying others. Even goldfish, some of the derpiest and friendliest species in the hobby, can start terrorizing smaller fish if their aquarium is small and/or overcrowded.

Respectively, zebra danios can fight one another in a smaller tank or nip the fins of long-tail fish, so providing them sufficient space, a good number of other danios to play with, and no tempting targets is a good idea.

Many of the measures suggested below are solutions regarding the aquarium setup.

How to Stop Fish Bullying Other Fish

Most of the below tactics can be used together. Please, read through all of them to understand what the implications of each action are and make the best decision for your particular case.

Proper Aquarium Stocking

One key principle to stopping fish from fighting other tankmates is proper stocking.

Don’t put aggressive, large fish among small, peaceful fish. The obvious exception here is the pea puffer which is small and cute, but the size is misleading. It will try to dominate territory, attacking any intruder quite viciously.

The same goes for bettas that have been raised alone.

Sometimes small fish, like the tiger barbs, can be quite feisty and will attack larger species.

The other side of stocking a community tank is to not overcrowd the space. The general rule is to keep an inch of fish (2cm) in two gallons of water (8 liters). It is a very broad rule that has many nuances, but I would always recommend to err on the side of fewer fish. Even if there is no bullying, overstocking the tank can pollute the water and stress the fish, leading to susceptibility to diseases.

If you want more livestock, add shrimp and snails. They are very interesting animals that keep things clean.

Now, let’s examine the best ways to deal with fish bullying.

Introduce New Fish at Night

Adding new fish when the aquarium lights are out reduces the chance of its inhabitants bullying the newcomers. Most fish sleep at night, including all species mentioned above. Only plecos are nocturnal, but they’d harass only other plecos if food is not enough.

Introducing a new fish at night allows it to find its spot and feel more comfortable by the time older tankmates become active. Instead of them seeing it “invading” their space, they’ll just find another fish already living in the area.

I’ve seen danios harassing new guppies right away under the lights. After turning off the light and re-introducing the new fishes in darkness, they found a way to co-exist with the hyperactive zebra danios without a problem.

Expand the Shoal or School

Some species, like the aforementioned danios, must live in groups. They are very social and build their hierarchies through somewhat aggressive play.

If you see the group picking up constantly on the weakest individual, introducing new members of the same species may alleviate the issue.

On the other hand, fish like oscars can be aggressive if they are alone in a tank. Adding a second fish of the same species, preferably of similar size, can balance things out.

Another way to deal with such situations, especially if the tank is already full, is rearrangement.

Rearrange the Tank

Changing things around can greatly reduce wars for territory. Fish protect the space they’ve claimed as theirs. Moving hardscape and plants around changes their world and forces the tankmates to re-establish their territories.

A more conservative approach is to add a new hardscape piece or a few more plants. It may provide sufficient hiding spots for the bullied fish. It certainly is worth a try, as it is a quicker and more immediate solution than total aquarium redecoration.

Visual barriers help with hiding. While fish use their entire bodies to perceive their environment, they form visual memories of their surroundings and use their eyes to spot other fish.

Reduce the Water Temperature

Fish are cold-blooded and colder water will slow their metabolism. Lowering the temperature within tolerable limits can help with overly aggressive behavior.

Be careful to lower it gradually to avoid cold temperature shock. Also, don’t go below the recommended degrees for the fish you are keeping.

Lowering the temperature has additional benefits — it slows down decomposition so uneaten food won’t cause ammonia spikes that easily. On the other hand, some parasitic illnesses like itch are more likely to occur in colder water.

Take Out the Bully

If none of the softer measures work, the bully may have to be taken out of the picture.

The easier way of doing this is to put the aggressor in a time-out basket. Keeping it there for 2-8 days will allow the free-swimming fish to rearrange their hierarchy and territory.

If the bully resumes its harassing behavior after its release, another spell in prison might redeem its character.

In case it doesn’t, you’ll have to consider a new home for it. Be that a new tank or return to the fish shop in exchange for another fish, you should find a solution. Otherwise, the bully will keep on attacking its victims and they’ll likely die.

Breeding Fish

A quick word on fish ready to breed.

Many species require special conditions to breed, but in a community tank, it may happen by chance. Providing livable water parameters for a variety of species could result in fish that are ready to procreate.

Zebra danios, for example, are among the easiest fish to breed and could spawn without concentrated effort on your part. But danios have zero paternal instincts. In fact, they’ll simply eat their own eggs without becoming overly aggressive toward other species.

Female goldfish, on the other hand, are rather protective of their eggs and will chase other fish.

Overall, breeding is best done in breeding nets or, ideally, breeding tanks.

What Is Fish Bullying and What Is Normal Interaction?

I mentioned danios many times throughout this article. They are a perfect example of fish that always tethers on the verge of aggressiveness. They chase each other around, pushing, running, dancing, and looking like maniacs.

By far and large, that’s their normal behavior. Usually, this playful aggression is spread out among the members of the group. Nobody is singled out and piled upon. As long as no zebra danio has broken fins or considerably paler colors, there’s nothing to worry about.

Male gouramis also could go at one another occasionally, but if the clashes are sporadic and short, all is good. But if one of them begins to exhibit signs of stress like damaged fins, lack of color, lack of appetite, or strange swimming patterns, then it is a good idea to intervene.Observe your fish and you will quickly learn when playful, healthy interaction transgresses into dangerous, harmful bullying. You already know how to deal with aggressive aquarium fish. Use the above tactics to stop fish bullying once and for all!

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