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Zebra Danio Killing & Chasing Other Fish: 6 Quick Solutions

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Zebra Danios are generally an excellent addition when it comes to freshwater fish tanks. However, I often caught them chasing, attacking, and even killing other fish in my aquarium. Over the years, I’ve learned several reasons that might force them to do so. I also came across a few tricks that stopped them from misbehaving. Now, I am willing to share that experience.

Zebra Danios usually chase and kill other fish due to inappropriate water conditions, including elevated toxins, low pH, and inadequate temperature. However, Danios may also attack their tankmates to establish a hierarchy or when the tank is overcrowded and features too few females.

As we move forward, I will share a few other reasons that may compel your danios to attack their tankmates. I will also take you step-by-step to solve it and eventually achieve peace in your aquarium.

Why do Zebra Danios Chase and Kill Other Fish?

Can Zebra Danio kill other fish? This question might surprise some people because Zebra Danios are social and mostly peaceful fish.[1] However, like most fish, the right (or wrong) conditions can change the behavior of your Danios, driving them to chase, bully, and ultimately kill their tankmates. 

Some factors that may contribute to this behavior include:

1. Your Danios are Exposed to Low Water Quality

Whenever a fish changes its behavior, you should make the quality of its water your first consideration. A fish’s health is entirely dependent on the quality of the water in the tank. When the water deteriorates, the fish’s health will suffer.

If the deterioration in water quality is significant, the fish will develop various diseases. If the poor conditions persist, it will die. However, if the deterioration isn’t significant enough to attract serious ailments, the water’s low quality will distress the fish. That will cause them to act in ways that are not wholly natural.

That includes inducing aggression in the creatures, and Zebra Danios are no different. They can survive in a wide variety of conditions, so you won’t kill them by only allowing a few of their parameters to deviate from the ideal.[2] But the resulting distress could push them to make the life of their smaller, weaker tankmates difficult.

2. Danios are Pretty Active

Zebra Danios are generally peaceful. However, they are also very active, which is why they spend a lot of time chasing one another around.[3] Sometimes, they do this for mating purposes. Yet, slower, less active tankmates will hate this behavior.

The danios don’t have to bully them directly. Their penchant for repeatedly zipping all over the tank will induce stress in the slower fish, and that stress can lead to death, especially when you consider how relentless danios can be.

3. The Tank is too Small

Because Zebra Danios are so playful, they need a lot of space. The more prolonged and extensive the tank, the better. If you force them to inhabit smaller, crowded tanks, they will lash out. This can also happen in aquariums with a lot of foliage.

Like most fish, danios prefer planted tanks. However, they also need open spaces. If you crowd their tank with plants, you could encourage them to act aggressively, mainly if they conclude that they need to protect their territory from their tankmates.

4. They Try to Establish Hierarchy

Zebra Danios are hierarchical creatures. Sometimes, they will engage in somewhat aggressive behavior to assert their dominance. This might look like infighting on occasion, but it isn’t dangerous. And for the most part, once the hierarchy has been set, the aggression will fade.

Such playfulness shouldn’t concern you. It is unlikely to concern the other fish in the water. However, new aquarists don’t know this, and they might overreact to this playfulness in their tank. Ultimately, unless you see signs of trouble such as nipped fins, you have no reason to worry.

5. There are too Few Danios

Danios are schooling fish. This probably became obvious once you realized how active and playful they are. The creatures typically chase one another, but they cannot do that if they are alone in the tank.

A single Danio could become a menace to the rest of your tank. Their isolation will induce stress, which can become aggression in the long run. Two danios are no better since they will stress one another out. The minimum number of Zebra Danios that should be kept in a tank is three.

However, that is if your circumstances have prevented you from adding more fish. Three is still too small a number. Two of the fish are more likely to gang up on the third one. Because they are so fast and active, you don’t want your Zebra Danios to be unhappy. They will make everyone else miserable.

6. The Tank is Overcrowded

Overstocking is just as bad as a small tank. If you crowd their environment with fish, the Zebra Danios will lash out. This is also true for other species. So if you don’t have enough danios, you cannot compensate by flooding the tank with other fish.

In overcrowded tanks, your Danios will encounter other fish more frequently. And the more they come across their tankmates; it is more likely that they will chase them. Sometimes that chasing ends up with fish that couldn’t survive.

7. They Share Their Tank With Gentle Tankmates

Even though Zebra Danios are peaceful, they have a reputation for nipping at other fish fins. This makes them a problem for fish with long, beautiful, trailing fins. Even in the absence of factors that induce aggression, Zebra Danios are likely to nip at those fins.[4] 

For that reason, species like angelfish and bettas should not be paired with Danios. You are likely to find their fins ripped because they are too gentle. And once a bite occurs, the targeted fish becomes prone to diseases and infections.

How to Keep Zebra Danios from Chasing Other Fish?

As you now know, it is perfectly normal for Zebra Danios to chase each other and even their tankmates. This comes naturally to them. However, if that behavior has evolved into bullying and violence, you can control your Zebra Danios using the following methods:

1. Adjusting the Water

If the danios are lashing out because of the tank’s conditions, I suggest improving the quality of their water. For that, I highly recommend getting the API Aquarium Test Kit (link to Amazon). That highly affordable bundle will accurately measure your pH, nitrates, nitrites, and ammonia within minutes.

There are cheaper solutions, although this kit lasts forever. You are not likely to run out of tests, no matter how frequently you perform them. If you notice that the toxins are too high, start by performing a water change. If the water quality is at fault, a water change will bring about a transformation in the danios’ behavior.

Once their distress is lifted, their violent behavior is more likely to stop. It would help if you also paid attention to the temperature (65 to 77 degrees F), pH (6.5 to 7.2), and hardness (Soft-Medium). Check the heater, filter, and pumps to ensure that they are working as expected.

Quite commonly, temperature fluctuations derive fish mad. For that, I highly suggest considering the Cobalt Aquatics Neo-Therm Pro Aquarium Heater (link to Amazon). That is the only device that keeps my water stable and my fish stress-free. I also reviewed it here.

It would be best if you also kept the water free of pollutants like dead plants and animals, as well as leftovers. Even though they can survive poor water conditions, the water quality can still drive Zebra Danios to act aggressively.

2. Getting the Right Tank Size for Danios

The fish can technically survive in 5-gallon tanks, although the minimum should be 10 gallons. To be on the safe side, you should start with a 20-gallon tank. The more room they have, the happier they will be. 

The tank should also be relatively long. That will provide your Danios with plenty of space to swim. If you suspect that your tank is too small, feel free to check my recommendations for aquarium kits. I even reviewed the precise kit that I use.

3. Adding a Few More Danios

If your Zebra Danios are misbehaving, you can dilute their aggression by adding more danios. As shoaling fish, they are happiest in groups. The bigger the group, the less likely the danios are to attack one another or harass their tankmates. 

A decent starting number is six or seven fish. Zebra Danios are easy to care for, so a large school won’t necessarily make life harder for you. As always, I would suggest paying attention to the tank size. Don’t add so many danios that you crowd your tank.

4. Considering the Right Male/Female Ratio

When it comes to female and male Danios, I suggest keeping two or three females in the tank for every male zebra danio. The easiest way to eliminate aggression is to keep the danios happy. A scarcity of potential mates will make the danios unhappy. 

Some males even fight one another, and the few females in the aquatic environment won’t fare any better. The attention they receive from the male danios will stress them, and it might even come off as bullying. This is why the tank needs a healthy number of females.

Some aquarists keep females out of their tanks altogether to prevent the males from fighting over them. While this can work, male danios are more than capable of chasing and frustrating one another in a tank that doesn’t have females.

5. Resetting Territories

If your fish keep fighting over the tank’s territory and you have determined that the tank size is sufficient, you can try rearranging the environment. Some people think that their fish are territorial when, in truth, the danio tank is too small.

But if that isn’t the problem, you can bring some much-needed peace to the tank. All it requires is to take the fish out, rearrange the items, and then put the fish back. With their old territories destroyed, the fish are more likely to behave.

6. Adding a Few Plants and Decorations

It would be wise to provide hiding places in the tank by adding plants, rocks, caves, and the like. Objects in the water create barriers that fish can use to steer clear of their bullies. If that doesn’t work, take the direct approach. 

Find the aggressive Zebra Danios and remove them. If you don’t want to lose the danios in question, you can add a divider that will keep the aggressive fish away from their tankmates. Some fish stores offer dividers, although you can build your own quite easily.

What Fish are Compatible with Zebra Danios?

Compatible fish for danios include docile species such as platys, guppies, cardinal tetras, loaches, and rasboras. Danios can also live with their kind, preferably with a ratio of three females for every male. Angelfish and bettas can get easily hurt by the danio’s bites and should be avoided.

  • Platys – Platys are easy enough to maintain. They have an average size of 3 inches. In other words, the size difference between Platys and Danios isn’t big enough to present a challenge. Like Zebra Danios, Platys are peaceful and playful.
  • Kuhli Loach – Kuhli loaches, which are roughly four inches, are nocturnal. They spend most of the day in hiding. As such, they are unlikely to clash with your Zebra Danios.
  • Cardinal Tetra – Cardinal tetras are just two inches. So they won’t pose a threat to your Zebra Danios or vice versa. They get along with their neighbors. Being schooling fish, they like the company. For that reason, the notion of sharing a tank with your danios won’t bother them.[5]
  • Guppies – This is another social species. At 2.5 inches, guppies are within the appropriate size range. They also thrive in groups. Your danio’s hyperactive behavior is unlikely to bother them.
  • Harlequin Rasbora – Harlequin Rasboras are small and attractive. They are known for their happy personalities. They will thrive in any tank that is free of aggressive fish.

Ultimately, when choosing tankmates, you have to prioritize non-aggressive species that are neither too small nor too large. However, it would be best if you avoided docile fish that cannot handle the energy of Zebra Danios.


If your Zebra Danio is chasing other fish, including other danios, your first step will be measuring the water parameters. That is because fish usually become aggressive once stressed, and the most common cause of stress is inadequate conditions.

Start by testing the water pH, ammonia, nitrites, and nitrates. If everything is okay in this section, make sure that the tank is not overcrowded. It would help if you also considered adding a few females to the tank. Three females for each male is about right. That should lower competition and aggression.