Are GloFish Aggressive? (& Why It Happens)

Last Updated on 2023-05-24

GloFish are beautifully colored fish that many people like for their aquariums because they add vibrancy to them. Many owners witness aggressive behavior from their GloFish, though. It raises concerns about the health of the fish, as well as the safety of its tankmates, and if the GloFish is aggressive by nature.

Aggressive behavior isn’t uncommon for GloFish. But it is best to figure out what the cause of the behavior is. This article is all about aggressive behavior in GloFish and what you should know about it.

Are GloFish Aggressive?

GloFish tend toward aggressive behavior. The potential is there. The level of aggression and the reason for it depends on what type of GloFish you have. Some types prefer to be part of a large group of fish, while others display more aggression in groups of five or more.

Do GloFish Fight Each Other?

GloFish do appear to fight each other sometimes. This happens because they’re territorial and want more space. It also happens because there are too few or too many of their own kind in the tank with them. These fish stress if they’re uncomfortable. They can take the stress out on other fish in the tank.

What Causes Aggression in GloFish?

One of the biggest driving factors for why GloFish act aggressively toward other fish is fear and loneliness. They may also want to show another fish who’s in charge of the space. Other causes of aggression include fighting over food and mating. Let’s take a closer look at some of these causes.

Fear and Loneliness

GloFish live with others of their kind in schools when in the wild. Schooling fish take comfort in having all of those other fish around them. Whenever a schooling fish doesn’t have that group to make it feel safe and secure, it gets scared and lonely, which often causes the fish to lash out at whatever other fish is around.

The scenario above can play out in your aquarium when you get only a few GloFish and put them in a tank with other types of fish. Three GloFish don’t constitute a school and won’t give your fish what they need to feel safe. A scared GloFish will almost always get aggressive with other fish.

Territorial Instinct

The territorial instinct is exaggerated in aquarium fish because they live in a tight space. Wild fish get territorial about their environment, also, but can eke out another living space if needed. There’s more space for fish in the sea or other bodies of water.

Aquarium fish will lay claim to an area of the tank. It’s usually somewhere that allows for hiding out. Should other fish come around the hiding spot, the fish who “lives” there will likely get aggressive. It’s easy to spot fish with a strong territorial instinct. They usually guard their territory closely.

Fighting Over Food

Most living creatures fight over food, including humans. GloFish are no exception. If there is a shortage of food, GloFish will behave aggressively toward other fish to secure their meals.

Just the perception that there won’t be enough food to go around is enough for a GloFish to get aggressive at mealtime. Fish tanks that are overstocked with fish often have occurrences of aggression between fish. You’ll also see a lot of fights over food in tanks with many large fish. Small fish are more aggressive when large fish are present at mealtime.

Fighting Over Mates

Most of the animal and aquatic kingdoms consist of species that fight over mates. Generally, the male GloFish will fight over females, not the other way around.

During breeding times, GloFish get even more territorial. If a male knows a female likes to hang out in a certain part of the tank, he will get more aggressive about defending his territory.

In cases where there aren’t many females for males to mate with, the aggressive behavior is worse. Males have been known to fight to the death over a female. For this reason, it’s best to stock your fish tank with more females than males rather than the opposite.

What Are Signs of Aggression Occurring?

How do you know when you’re witnessing aggressive behavior from a GloFish? You may notice chasing, nipping, splits of fins, varied territories, scratches, missing scales, or changes in the behavior of some fish.


Chasing is the most obvious sign of aggression between fish. It is just what it sounds like; one fish swims around after another in a harassing manner. Chasing indicates coupling, dislike, food insecurity, or a battle over territory.


Nipping isn’t always about being aggressive. Nipping can also indicate an attempt to mate. Male fish will sometimes nip at females to show an interest in mating. Females may nip at fish who get too near to their eggs or fry. Mother fish are protective over the territory where they laid their eggs and where they keep their fry after hatching. They nip at other fish to warn them off.

Split Fins

You won’t always see aggressive behavior occurring in your aquarium. Many fights happen behind the scenes. What you will notice is evidence of a fight having taken place. Split fins are usually good indicators that a fight has occurred.

A split fin isn’t a serious wound, usually. Many fish nip at the fins because they intend to show dominance or give a warning, not cause harm. It may just mean you need to do something about the tank environment to ensure there’s enough space for all of your fish to feel comfortable and safe.

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Varied Territories

If you notice fish taking up residence in another territory, it may indicate there is aggressive behavior taking place. Multiple fish may be getting together and “bullying” another fish. The “bullied” fish often gives in and moves to another part of the tank. While this may seem harmless, it’s something you’ll want to pay attention to and address, if possible.

Scratches or Cuts

As we mentioned above, some fights are fights to the death. That also means some fights between fish do result in serious injuries. Look for scratches or cuts on the bodies of your fish. If you find any, you may need to make some adjustments in your tank environment or even change out tankmates.

Missing Scales

An interruption in the scales on a fish’s body can indicate several things, but one of them is aggressive behavior between fish. Do a little investigating into who the aggressor was. It may be that the fish with the missing scales isn’t the victim. If that’s the case, leaving that fish in your tank is potentially putting all of your other fish at risk.

Changes in Behavior

Fish owners get familiar with their fish. Most of us like to observe our fish throughout the day and watch their different behaviors. Fish, like humans, tend to act a certain type of way. They have their own personalities.

Sometimes, fish will begin behaving in a way that doesn’t seem normal. The more time you spend observing your fish, the better you’ll be at identifying different behaviors. It makes it easier for you to know when your fish have a shift in routine and behavior. Changes in behavior often indicate aggression exists in the tank.

How Can You Stop GloFish Acting Aggressive?

The steps you take to stop the aggression from your GloFish depend on what is causing it.

How to Fix Territorial Aggression

If there is a territorial issue, you should increase the size of the tank and give your fish more places to hide. It may be the fish just feel it’s too crowded in their world.

A new tank may not be a feasible option for you. If it’s not, try rearranging the items in the tank to confuse the aggressive territorial fish. Sometimes you just have to shake things up a bit.

How to Fix Targeted Bullying

Some fish don’t want to be alone. A fish who is the only one of its kind in the tank may get picked on by the other fish. Sometimes all it takes to rectify this situation is another fish of that species.

How to Fix Fear Aggression

It can be that the lonely fish is the aggressor in the tank. This is especially true for schooling fish. The lonely, scared fish needs to have more of its kind added to the tank.

How to Fix Mating Aggression

You’ll likely have to tolerate some mating aggression. It’s part of the process. However, you can reduce the level of aggressive behavior between males by adding more females to the tank.


Are GloFish Fin Nippers?

GloFish are known to nip the fins of other fish. You’ll see this a lot when there are only two or three GloFish in your tank.

Is It Normal For GloFish To Chase Each Other?

GloFish chase each other for many reasons. They chase to show aggression when trying to mate and to protect eggs, fry, or territories.

Wrap Up

GloFish are aggressive for various reasons. Before adding them to your tank, educate yourself on what fish make the best tankmates for GloFish. Make sure you give them enough other GloFish to school with so they feel safe. Don’t overcrowd the tank and address any “bullies” as soon as possible.

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