Are Glofish Aggressive?

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Glofish are a unique group of fish that are most popular thanks to their fluorescent appearance. This can make some people nervous since a lot of them believe that the fluorescent colors are the result of injections and dying, which isn’t true.

Glofish are the result of genetic engineering which aimed to produce fish capable of glowing in the presence of human-produced water toxins.

The result was a glowing fish that everybody loved, which led to them being produced in mass.

You can now purchase them from qualified shops and I say ‘qualified’ because Glofish are still the result of patented technology. So, you can own them, but not breed or sell them.

That being said, are Glofish a good fit for community tanks? How aggressive are they and can you mitigate their aggression if necessary?

Let’s see which species we are talking about and discuss their aggression level.

Which Glofish Are the Most Aggressive?

At this moment, there are 5 different species of Glofish available, with more on the line to receive the same genetic treatment in the long run.

Check the table below:


Fish Species


Aggression Level


Additional Information

Zebra Danio Moderate
  • Zebra Danios are more aggressive when not in the presence of other members of the same species.
  • This is a schooling fish, which means it needs the company of its own to remain calm, healthy, friendly, and active in the long run.
  • You should keep at least 6 Zebra Danio in the tank. Any less than that will make them more aggressive, causing them to nip at other fish’s fins and stress them out.
Tiger Barbs Moderate/High
  • Tiger Barbs aren’t particularly territorial, but they have very strong hierarchies to attend to.
  • This means that they will fight each other, males in particular, and other fish that may pose a threat to their dominant status.
  • Tiger Barbs are also schooling fish that are calmer and more peaceful in larger groups.
  • Otherwise, they may display extreme aggression towards their tank mates. In severe cases, Barbs kept alone may attack and kill other fish in their habitat.
Neon Tetras Low
  • Neon Tetras are non-aggressive fish that will go along great with any tank mates, provided they are also peaceful and friendly.
  • The problem is that Neon Tetras can sometimes display aggressive behavior which is mostly triggered by poor environmental conditions.
  • Tetras will fight among themselves and nip at other fish’s fins if they are stressed, unwell, or kept alone or in duos.
  • Provide them with optimal water conditions and regular tank maintenance to prevent that problem.

Rainbow Sharks


High/Very High

  • Rainbow Sharks are as aggressive as they are beautiful.
  • They need around 30 gallons of water per specimen to feel comfortable in their environment and they are extremely territorial.
  • Rainbow Shark males will fight each other to the death and they will attack any other fish entering their territory.
  • You should either keep them solo in the tank or pair them with fast swimmers that can avoid them.
  • Also, provide the tank with a lot of plants and hiding areas to minimize their aggression and provide their potential victims with reliable escape routes.
Betta Fish High
  • There’s a reason why Bettas are known as the Siamese Fighting Fish.
  • Male Bettas often display extreme territorial behavior and will fight each other relentlessly if not given enough space.
  • They will also attack other fish for the same reasons and display extreme aggression during the mating phase.

This table shows that different fish species display different aggression levels for different reasons.

If we were to choose several common denominators, those would be:

  • The lack of proper space
  • Fish stress is caused by overcrowding, poor feeding, inadequate water parameters, or bullying
  • Natively elevated territorial behavior
  • Incompatibility with community-based environments

As a general rule, Glofish display different behaviors and personalities, depending on the species they belong to.

Signs of Aggressive Behavior in Glofish

As a fish keeper, identifying the early signs of aggression is a key ability, since it can prevent more serious issues along the way.

Unsupervised and unmitigated aggression can lead to injuries, continuous fish stress, and even death.

Here are some of the widespread reasons for aggression in tank fish, Glofish included:

Chasing Each Other

This is a tricky one since most fish will chase each other in a playful manner, as well as during the mating phase.

They also display similar behavior when enforcing hierarchical rules, during which dominant males or females put lower-ranked fish in their place.

The chasing behavior itself isn’t necessarily a sign of unwarranted aggression.

The problem occurs when the aggressive behavior seems to have become the norm. If your fish chase each other constantly around the tank, that may be a sign of aggression.

At this point, you need to identify the cause, pinpoint the aggressor, and try to mitigate the situation before devolving into murder.

Constant chasing may lead to fish stress which lowers the victim’s immune system and makes it more prone to disease and infections.

Fin Nipping

The fin nipping behavior is a staple to some species like the Zebra Danios, but it’s almost always triggered by something.

Many fish nip at each other’s fins when playing, mating, imposing territorial order, or for hierarchical reasons. Your job should be to assess the situation, understand what fuels the behavior, and find a solution.

The fish nipping behavior isn’t immediately a threat, but it will become a problem in the long run.

It can lead to injuries that risk infecting and will stress out the victims, promoting hiding behavior, a poorer immune system, and an overall tensed environment.


This is about all-out violence that many fish species may indulge in occasionally.

There are several reasons why Glofish would resort to violence, including territoriality, hierarchical fights, mating, food competition, and even the innate aggression towards any fish entering their habitat.

The rainbow shark fits the latter category, for instance.

The problem here is that violence itself isn’t the real issue. The real issue is repeated violence or the type of violence that reaches higher-than-normal levels.

Since we already mentioned rainbow sharks, let’s keep using this example.

Male rainbow sharks may display extreme aggression towards slower and more peaceful fish species entering their territory and they won’t go easy on them.

It’s documented that rainbow sharks will even kill their tank mates and their aggression is even higher during the mating phase.

This is why you shouldn’t keep Glofish rainbow sharks with any other fish species and never have more than 1 male in the same tank.

The same goes for any fish species that displays abnormally high aggressive tendencies.

As a general rule, fighting isn’t bad in and of itself. As I’ve explained, fish will resort to violence primarily to create order in their community and establish specific hierarchies.

It’s repeated or excessive violence that should concern you.

If the aggressor cannot be tamed, consider removing it from the habitat to protect the rest of the fish.

If that’s not possible, at least decorate the tank with a lot of fluffy plants, caves, rocks, or wood to break the line of sight between the fish.

This will provide fish with hiding areas where they can feel safe and use to avoid their aggressors.

Why Do Glofish Become Aggressive?

The reasons for Glofish becoming aggressive usually differ from one species to the next, but not by much.

Typically, all fish species will fall into one or more of the following categories in terms of aggressive behavior:


The mating season (or phase in some species like guppies which don’t have a mating session, since they mate all the time) is a great opportunity for creating offspring and being violent towards one another.

The male Glofish will compete over the opportunity to mate and they’re almost never nice to each other.

Male Bettas, for instance, are notoriously violent and may even kill each other if they don’t have enough females to go around.

This is why it’s recommended to keep at least 3 females for each male Betta. This will minimize the aggressive display and ensure a calmer and friendlier environment.

So long as we can consider ‘calm’ and ‘friendly’ accurate descriptors of an environment that houses several male Bettas.

The same solution applies to all Glofish. Having a healthy female-to-male ratio is key to minimizing or even preventing violent behavior during mating.

Another problem relating to this aspect is female aggression. Female Glofish may sometimes become aggressive themselves if they’re subject to constant pushes from males.

Male Glofish don’t know when to stop and accept that the female won’t mate with them. This will cause them to stress the female and, in turn, cause the female to attack them.

This contributes to a tense environment that’s unfit for females anymore.

Feeding Time

Feeding time is always an opportunity to reinforce hierarchical order. Dominant males and females will keep the rest in line during feeding and fights may break out occasionally.

This tends to happen if there’s not enough food to satisfy all fish.

A problem that may arise with that is weaker fish fasting or eating less than others. You should also feed your Glofish regularly and provide enough food for everyone to get their share.

You can achieve that by feeding the fish more often (twice per day should do) and spreading the food all over the water’s surface.

This strategy will allow all fish to get access to food and lower food-related aggression as a result.

Lack of Mates

Glofish tend to be friendlier, more active, and more peaceful in groups, but that’s not a universal rule. It applies to neon tetras and zebra danios, but it doesn’t apply to Bettas or rainbow sharks, for instance.

When discussing those that the rule does apply to, try to keep them in larger schools if possible.

This will minimize their aggression as joining a group of same-species fish will keep them entertained and mentally balanced.

For zebra danios and tetras, the school should consist of at least 6-8 specimens. You can add more if they live in a single-species tank.

Overcrowded Tank

Overcrowding is one of the primary causes of fish aggression. This problem is so prevalent that it may trigger aggression in fish species otherwise peaceful, like guppies.

The main reason for that is males’ territorial instincts that will be put to the test in an overcrowded environment, where they will constantly bump into other fish.

In species like Bettas, rainbow sharks, and tiger barbs, overcrowding can have fatal consequences fast. Male bettas will definitely fight to the death if not given enough space, but fighting isn’t the only issue.

Overcrowding also leads to more fish waste and food residues burying in the substrate and decaying out of sight.

These will increase ammonia levels dramatically, soon poisoning the entire fish population.

An interesting aspect is that some fish species, like Mbuna cichlids, don’t mind overcrowding; they actually thrive in an overcrowded environment.

The more of them there are in the same habitat, the calmer the cichlids (males especially) will be. This doesn’t apply to Glofish.

To prevent overcrowding, learn each species space requirements. A rainbow shark needs around 30 gallons of water, while 1 betta fish requires at least 3 gallons of water.

Try to provide your fish with optimal living conditions and sufficient space to minimize their aggression and stabilize the population dynamics.

Small Tank

A small environment will also irritate some fish species, especially larger ones. Pretty much all Glofish species are inquisitive by nature and like to roam their environment as much as possible.

A small habitat will stress them out and cause them to become lethargic, depressed, and even violent.

If you’re investing in Glofish, tank equipment, and quality fish food, you might invest in a larger tank as well.

Provide your fish with proper space and water volume and check their interactions to watch out for signs of aggression.

A larger tank is necessary especially for schooling fish or community settings. You will need the extra space for tank equipment like a heater, air pump, or filter, and sufficient room for some aquascaping.

Glofish live their best lives in natural-looking habitats filled with plants, adequate substrate, rocks, wood, and other aquatic decorations that would mimic their natural environment.

The idea is that a larger tank allows you to experiment with different setups, whereas a small one won’t. Plus, you will only need to buy the tank once, so you might as well get a larger one from the get-go.

This will spare you of the need to upgrade it later on, when your fish start reproducing or if you decide to buy more Glofish.

Do Glofish Kill Each Other?

It can happen, yes. There are 2 scenarios where the interactions between your Glofish can turn deadly.

The first is where you keep multiple male Glofish belonging to the same species in the same environment.

The more males there are, the higher the risk of lethal aggression. Bettas are the most compelling example since male Bettas will often fight to the death over territory dominance and mating rights.

The same applies to tiger barbs and rainbow sharks.

The second scenario is when you combine incompatible fish species that have no business being in the same environment. Keeping bettas with rainbow sharks, for instance, is a recipe for disaster.

While they don’t share the same living space, both species are highly territorial and aggressive and are bound to wage full-on war against each other. With obvious dire consequences.

The solution is simple and obvious in both cases. Limit the number of males and don’t mix incompatible fish species.

Can Glofish Live with Other Fish?

Yes, but it all comes down to the ‘species compatibility’ issue that I was mentioning earlier.

Technically speaking, there’s no real difference between a normal betta and its Glofish version, other than the fluorescent glow, of course.

Other than that, they are the same in terms of behavior, water requirements, diet, personality, etc.

So, there’s no problem with mixing regular and Glofish betta, so long as you don’t mind breeding them.

The Glofish gene only passes on to the fry if both parents have it. So, a betta Glofish mating with a regular betta won’t result in Glofish fry.

Are Glofish Mating or Fighting?

The line is pretty thin between mating, playing, and fighting when it comes to fish in general. The mating process may look a lot like fighting to the untrained eye, but you can tell the difference if you look closely.

The first sign that you may be looking at mating attempts is if the interactions take place between a male and a female.

Males and females rarely fight amongst themselves.

Depending on the species, the mating process may differ slightly, as each male seeks to impress the female and convince it to breed. This process may involve chasing, touching each other, and even poking and fin nipping at times.

The female may reject the male at first and try to outrun it but, eventually, the male’s persistence will pay off.

Contrary to that, fighting fish will display different behavior. They will snap at each other, put more force behind their pokes, and display increased aggression that’s unnatural for the mating process.

A clear indicator that your fish are fighting is if they’re both males.


Glofish can be aggressive under given circumstances, but there are ways you can mitigate that aggression.

It all begins with learning as much as you can about your favorite Glofish species and providing it with optimal living conditions.

Give your fish enough space, decorate their environment to resemble their natural habitat, limit the number of males, and monitor their population dynamics.

Some aggression is normal from time to time, as most fish species live based on hierarchies.

You only need to intervene if their aggression goes off the charts and threatens their wellbeing.  

Author Image Fabian
I’m Fabian, aquarium fish breeder and founder of this website. I’ve been keeping fish, since I was a kid. On this blog, I share a lot of information about the aquarium hobby and various fish species that I like. Please leave a comment if you have any question.
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