Fish Bullying and How to Address It

Fish bullying is a surprisingly common issue that aquarium owners face. In the confined spaces of home aquariums, some fish become aggressive and attack, harass, or intimidate other fish. Understanding the causes of bullying and learning to curb aggressive behavior is essential for maintaining a healthy and peaceful tank environment.

Fish Bullying

Why Does Fish Bullying Happen?

Fish bullying occurs for various reasons rooted in natural fish behavior and instincts. Here are some of the main causes of aggression and bullying among aquarium fish:

Territorial Behavior

Many species of fish are highly territorial, especially as they reach maturity. They will claim certain areas or decorations in the tank as their space and aggressively defend it from other fish. Cichlids, bettas, and discus fish are notorious for their territorial nature. This behavior is amplified during breeding seasons as they try to create and protect nesting areas.

Breeding Season Aggression

The breeding season triggers a major shift in fish behavior. Aggression, territorialism, and bullying spike dramatically as fish try to attract mates, claim nesting areas, and guard their eggs or fry. This is especially pronounced among egg-laying species like cichlids and bettas. The breeding aggression can last for weeks, resulting in relentless attacks on subordinate fish.

Food Scarcity

When food resources are limited, competition understandably increases. Fish will ferociously guard food drops and attack any fish that approaches. Omnivorous fish like cichlids and tropical community fish are more likely to bully when denied adequate amounts of food. Underfeeding leads to heightened food aggression.

Personality Clashes

Like people, some fish have naturally pushy or aggressive personalities that lead to bullying. Certain individuals will constantly harass and nip at others, refusing to back down or give their tankmates space. These “bully fish” assert their dominance through intimidation and rough play. Some common aggressive species include Oscars, Jack Dempseys, Green Terrors, and Convict Cichlids.

Types of Fish More Likely to Bully

While any fish can become a bully in the right circumstances, these types are most prone to aggressive behavior:

Size Doesn’t Always Matter

Small fish can terrorize larger tankmates if they are pushy and relentless. For example, the tiny Neon Tetra is known to fin nip placid species like bettas and angelfish. Groups of small fish may “gang up” on a single larger fish. So fish size alone doesn’t determine bullying likelihood.

Notorious Bully Species

Some fish species have earned a reputation for aggressive tendencies. Cichlids are notorious bullies due to their territorialism and breeding aggression. Dwarf gouramis also frequently terrorize other fish. Plecos, Oscars, Jack Dempseys, and Convict Cichlids are other examples of notoriously pushy fish.

Predatory Fish

It’s in their nature to hunt, so predatory fish often attack and harass tankmates. African cichlids, Arowanas, Snakeskin Gouramis, and Green Terrors are predatory fish with higher probabilities of aggression. Tankmates should be chosen carefully.

Distinguishing Bullying From Mating Behavior

It’s important to identify bullying behavior accurately. Some mating-related activities can initially look like bullying but serve a natural purpose. Here’s how to tell the difference:

Aggressive Mating Displays

Male fish pursuing or displaying for females may charge at and chase other males. But this ritualistic aggression ceases after mating. True bullying is frequent, ongoing harassment.

Guarding Behaviors

Fish defending eggs or guarding fry protect their offspring, not intentionally bullying others. These defensive behaviors are temporary until eggs hatch or fry mature.

Check for Harm

True bullying results in physical damage like torn fins, scales, or wounds. Occasional chasing during mating is harmless. Watch for signs of injury to distinguish bullying from breeding behaviors.

How to Stop Fish From Attacking Other Fish

If you confirm persistent bullying in your aquarium, here are some solutions to restore peace:

Increase Tank Space

Crowding stresses fish and promotes aggression. Adding more gallons per fish reduces territorial clashes. For community tanks, stick with 1 inch of fish per gallon of water.

Rearrange the Tank

Altering decorations and plants disrupts established territories. After rearranging, fish must reestablish domains, distracting them from bullying. Change things weekly.

Add More of the Same Species

For schooling species like tetras or barbs, increase group size to 6+ fish. More individuals spread aggression and prevent targeting of individuals.

Use Tank Dividers

Insert acrylic dividers to separate feuding fish while allowing water flow physically. Use as a temporary timeout to curb bullying.

Provide Hiding Places

Give bullied fish ample hiding spots like caves and dense plants. This allows them to retreat and reduces stress.

Remove Aggressive Fish

Rehoming relentless bullies may be necessary for a harmonious tank. But first try the above solutions before resorting to removal.

Should An Aggressive Fish Be Removed Permanently?

Removing an aggressive fish is a last resort but sometimes warranted. Here are guidelines on when to consider it:

No Improvement After Attempted Solutions

Removal may be needed if increased space, rearranging, more schooling fish, etc. fail to reduce bullying. Persistent harassment jeopardizes health.

Repeated Severe Injuries

Bullying resulting in deep wounds, torn fins down to the flesh, or rapid health decline indicates removal is best for the bullied fish.

Compromised Quality of Life

Bullied fish afraid to come out and constantly hiding signals an unacceptable quality of life. Removing the bully or bullied fish may be most humane.

Consider Rehoming Before Euthanasia

Try finding a new home for the aggressive fish before resorting to euthanasia. Shelters or local fish stores may adopt unwanted fish. Rehoming is preferable if it’s an option.

Signs of Fish Bullying

Learn to recognize the physical and behavioral clues that signal bullying in your tank:

Physical Signs on Bullied Fish

  • Torn or missing scales
  • Shredded or clamped fins
  • Open wounds or ulcers
  • Rapid gill movement and panting
  • Hiding near surfaces and corners

Behavioral Changes to Watch For

  • Afraid to leave hiding spots
  • Darting and dashing around tank
  • Clamping fins close to body
  • Lethargic movement and loss of appetite
  • Hovering at top of tank away from others

Special Cases of Fish Bullying

Certain popular aquarium fish are more prone to specific types of bullying. Here are some special bullying cases to be aware of:

Koi Fish Bullying

Koi are social but get aggressive in smaller tanks. Frequent water changes and 150+ gallon ponds curb bullying. Crowding stresses Koi as they grow.

Angel Fish Bullying Other Angelfish

Angelfish may bully each other as they mature, especially during spawning. Keep pairs or groups of 6+ and provide cave hiding spots in the tank.

Molly Fish Bullying

Placid mollies get aggressive if overcrowded. A 30-gallon tank, dense plants, and reducing male/female ratios prevents molly bullying.

Fish Bullying Snails and Other Non-Fish Creatures

Some fish pester and nip slow moving creatures like snails. Keep appetizing tankmates separated from aggressive fish.

Fish Bullying New Tankmates

It’s common for existing fish to bully new additions. Rearrange decor, dim lights, and gradually introduce new fish to ease the transition.

How to Stop Discus Fish Bullying

Discus establish social hierarchies via bullying. Reducing water changes, feeding 2-3 small meals daily, and having 5+ discus eases aggression.

How to Stop Parrot Fish Bullying

Parrots bully during breeding seasons. Rearranging rock caves, filtering aggression, and target feeding curb parrot cichlid bullying.

What Does Fish Bullying Look Like?

Here are common bullying behaviors to watch for among different fish:

  • Chasing and nipping fins (Smaller tetras and barbs)
  • Ramming and flaring gills (Bettas, gouramis)
  • Mouth fighting and lip-locking (Cichlids, parrot fish)
  • Tail nipping and damaging fins (Goldfish, koi)
  • Darting/lunging attacks (Predatory fish like Oscars)
  • Group swarming/harassment (Schooling fish ganging up)

Preventive Measures Against Bullying

Here are some tank management tips to help prevent bullying from occurring:

Careful Selection of Tankmates

Avoid pairing aggressive species like cichlids. Learn compatibility before selecting fish. Introduce docile fish first.

Proper Aquascaping

Create broken sight lines and territorial markers. Use caves, driftwood, and tall plants to obstruct bullying chase routes.

Adequate Filtration

Overcrowding and poor water quality spark aggression. Oversized filters and frequent water changes reduce stress.

Monitor Water Parameters

Test for ammonia, nitrites, nitrates, pH, and temperature regularly. Stability in water parameters prevents stress responses like bullying.

Quarantine New Fish

House new fish separately for 2-4 weeks to check for illness before adding to main tank. This prevents disease spread and bullying.

Feed Small Amounts Multiple Times Daily

Feeding only once a day ignites food aggression. Give smaller portions 2-3 times to minimize bullying at feeding time.

Supervise Initial Meetings

Monitor all fish closely for several hours after introducing new tankmates. Be prepared to remove aggressors.

Provide Ample Hiding Spots

Plants, rocks, and caves allow bullied fish to hide and establish security zones. This diffuses chasing and conflict.

Thought-Provoking Questions About Fish Bullying

Fish bullying elicits some thought-provoking questions about aquarium management best practices. Here are a few worth considering:

Is Fish Bullying a Sign of an Unhealthy Aquarium?

Frequent bullying could signify issues like:

  • Poor water quality
  • Improper pH or temperature
  • Overcrowding and lack of space
  • Not enough hiding places

If aggression persists after correcting these issues, there may be a fundamental incompatibility among the fish species selected. Rehoming specific fish may be needed.

How Effective Are Commercial Anti-Bullying Products?

Some products claim to reduce aggression by altering fish hormones or releasing pheromones. However, there are no magic cures for bullying. At best, these products might provide temporary distraction or relief. Improving overall tank conditions is more important for a long-term harmonious environment.

Ethical Considerations: Is It Right to Isolate an Aggressive Fish?

While removing a bully may be necessary to protect other fish, permanent isolation raises some ethical concerns regarding fish quality of life and adhering to natural behaviors. Intermittent timeouts and temporary partitions might be preferable alternatives in some cases. There are no easy answers, so focus on improving tank conditions first before resorting to permanent fish segregation or rehoming.


Fish bullying stems from natural territorialism, mating behaviors, hunger, and personality differences. While sometimes unavoidable, the negative impacts can be minimized through proper tank size, aquascaping, species selection, and monitoring for early signs of aggression. Relocation or removal of bullies should be a last resort after attempting other means of peaceful dispute resolution. Maintaining harmony requires understanding the complex social dynamics below the water’s surface. A well-managed tank allows even notoriously aggressive species like cichlids to cohabitate through proper care and planning.