Are Rasboras Aggressive?

Generally speaking, Rasboras are a non-aggressive species, and it is out of character if you see a rasbora acting aggressively. However, there are times when they will show aggression or try to establish dominance, like when they are spawning or when a new dominant male is introduced to the tank.

One of the problems people have when deciding what type of fish to add to their tank is what species are not aggressive. No one wants to have different groups of their fish attacking each other. That’s why it’s important to know as much about the species you add to your tank as possible.

Are Rasboras Aggressive?

Rasboras are a non-aggressive species that will get along well with other non-aggressive species, such as Mollies, Guppies, Tetras, and Danios. Rasboras are small, active fish that should be housed in a large tank.

Rasboras are not typically aggressive, but there have been instances where owners have observed their rasboras get more aggressive over time. There is usually a good reason and solution for this uncharacteristic behavior.

What Are The Signs Of Aggressive Behavior In Rasboras?

Although it is doubtful that your rasboras will show any signs of aggression, there is always that off chance something might change their behavior. That is why you need to know what aggression looks like in rasboras.

  • Charging other fish
  • Scrapes or scratches on the skin
  • Scales that are missing
  • Split or injured fins
  • Nipping at other fish

You may need to take action when you see any of these signs of aggression. Human intervention is the best way to get the fish to stop their aggressive behavior.

What Can Cause Aggression In Rasboras?

When you have a non-aggressive species like the rasboras become aggressive, you must find the reason so you can stop it immediately. Here are some of the main reasons rasboras can act aggressively.

Harlequin Rasbora In Front of Plant Leafs with Algae

Changes In Their Territory

While rasboras are not typically territorial, some males and even females may take exception to the rule. For example, it can happen when you move your rasboras to a bigger tank or you add another fish species to the tank. Although it is rare, these changes in the rasboras territory can trigger aggression.

The rasboras will show some signs of aggression, and you will need to remove the aggressive rasboras and put them in a quarantine tank until it starts to settle down. However, rasboras are extremely social fish and don’t handle being alone too well, so before you know it, your hulk fish should be back to its calm self in no time.

Spawning Time Can Trigger Aggression

While aggression is common in most animal species during their mating season, it’s rare to find it in rasboras. It is a possibility, though. Males will try and show off in front of potential female mates and charge their competitors to get the upper hand (or fin) and impress the female with their strength and vigor.

This form of aggression typically sorts itself out as soon as spawning season ends, and your fish should return to their usual calm state.

Sickness In Rasboras Can Cause Aggression

Changes in your rasboras behavior like increased aggression can be because it’s sick. When we don’t feel well, we try to keep to ourselves and can become easily irritated and snippy towards others. Rasboras are the same; they may act aggressively toward others if they feel sick.

If you suspect your rasbora might be aggressive because it is sick, then look for other symptoms to confirm your suspicions like,

  • The rasboras may float to the top of the tank a lot
  • A loss of appetite is a clear sign something is up
  • When a rasbora looks lethargic or swims slower than usual
  • Look for injuries to their bodies as fungal infection can cause fin rot

If you see any of these symptoms besides aggression, your rasboras might be sick and should get veterinary advice immediately.

Will The Wrong Dominant Male Intensify Aggression?

It is doubtful that adding a new dominant male might trigger aggression in rasboras, but it has happened. When the dominant male in a rasboras group dies or is removed, one of the remaining males may want to take up the leadership reigns.

It will nip fins to assert dominance and may pick on a weaker, non-dominant male. As a result, the other males might become more aggressive, or they will be subdued by the aggression shown by the male vying for the top spot.

Does The Number Of Rasboras In The Tank Affect Aggression?

The number of rasboras has a profound effect on their behavior. When you don’t have enough rasboras in the tank, they might start to feel territorial and begin to act out. By adding more rasboras to the tank, their aggression will decrease.

Adding a semi-aggressive fish species to the tank will also mean some fighting and the aggression in rasboras increasing. To keep the peace, avoid adding any aggressive fish to the tank.

How To Decrease Aggression In Rasboras?

When you find that your usually peaceful rasboras are acting aggressively, you must fix the situation before the fighting leads to severe injury. Here are a few ways you help decrease aggression in your rasboras.

  • Getting a bigger tank might help your rasboras keep calm. They are very active and get frustrated when they don’t have enough space to swim around.
  • Separate the troublemakers. When you have one aggressive fish, separating it from the group will calm it down. Rasboras can’t stay alone for long, and they should relax once you put them back in the tank.
  • Check for sickness and treat it immediately. When you find no other issue, your rasboras might be sick. You must have it looked at and treated immediately.
  • Add more rasboras to the tank. They might get frustrated and act out when you don’t have enough rasboras in your tank. Adding more rasboras to the tank and their group should mellow their mood.


Rasboras are not an aggressive fish species and get along well with other non-aggressive species. Unfortunately, there are times when they might become aggressive, but with care and understanding, you can get your rasboras back to their peaceful nature in no time.