Why is My Fish Bullying My Other Fish?

Aquarium hobbyists know that keeping pet fish is not just about creating a pretty fish tank. It also involves understanding fish behavior and ensuring their welfare.

One common problem that fish keepers encounter is fish bullying, where one fish repeatedly harasses or attacks another. This behavior can lead to physical injuries, stress, and even death in pet fish.

In this article, we will explore the causes of fish bullying, how to identify and prevent it, and steps to take when it does occur.

Understanding Fish Bullying

Fish bullying is a form of aggression that occurs when one fish repeatedly harasses or attacks another fish. You may observe one fish chasing others around the tank or fins tattered by biting. In some circumstances, the “victim” of the aggressive fish behavior can even die due to stress or injury.

Some fish species are more likely to engage in aquarium bullying than others. Cichlids, barbs, and male bettas are common culprits. As the aquarist, it is up to you to ensure proper tank mate compatibility.

Don’t make the mistake of labeling your aggressive fish as “bad.” They are simply responding to their environment and survival instincts. But there are things that you can do to mitigate fish aggression. We will discuss those steps below.

Identifying Fish Bullying Behavior

An important aspect of aquatic pet care is observing your pets regularly. In this way, you will stay aware of fish health and the signs of bullying.

Physical signs of bullying include missing, tattered, or split fins, missing scales, red blotches on the body, or any other physical injuries. Fish stress caused by bullying can also manifest itself physically, even when no injury has occurred. A stressed fish may lose its color and keep its fins clamped tightly against its body.

Aggressive behaviors to watch for include chasing and aggressive feeding. Some chasing or nipping of fins is the normal result of fish territoriality, but if it occurs constantly or results in injury, this is a danger to fish safety.

You should also look for changes in fish social behavior. Depending on the species you keep, it may be normal for there to be one dominant fish that gets first access to food and the best hiding spots. Fish breeding may also cause behavioral changes. But if your once-active fish is now refusing to come out of its hiding spot, it may be time to consider whether one of the factors listed below is leading to bullying within the tank.

Causes of Fish Bullying

Overcrowding and competition for resources are common causes of bullying. Overcrowding in fish tanks occurs when you, the keeper, place more fish in the tank than is sustainable – or when your fish reproduce. Hungry fish may compete for food and hiding places. Water quality can also suffer, leading to even more stress.

A lack of hiding places may be due to overcrowding or insufficient aquascaping. Fish tank setup involves the construction of aquatic ecosystems that mirror those the fish may encounter in the wild. You should endeavor to understand the aquatic ecology of the places your fish come from so you can adequately meet their needs.

Compatibility issues are also common in community tanks housing multiple fish species. Poorly matched tank mates may bully one another because they differ greatly in size or aggression.

Sudden changes in environment or routine can also affect fish psychology. A stressed fish may bully others to obtain more access to food or territory. After all, chasing and nipping are forms of fish communication.

Genetic predispositions to aggression can also play a role. For example, the Siamese fighting fish or betta was bred for more than a century to do what its name suggests – fight other fish to the death. The winners of such contests produced the next generation, passing on aggressive traits and physical prowess. The result is that today, no amount of tank space or ample hiding spaces will prevent two male bettas from bullying one another.

Effects of Fish Bullying

Excessive bullying will inevitably affect fish health. Bullied fish may weaken due to malnutrition. Stressed fish may lose their color and display disturbing changes in behavior. Fish disease is more likely in a stressful aquarium environment, and your fish may die prematurely. The effect on you as a hobbyist can be stress or concern for your pets and decreased enjoyment of fishkeeping.

But there are things you can do to prevent bullying from occurring and respond when it does occur.

How to Stop Fish Bullying

Avoiding Overstocking

Every aquarist wants a beautiful, active tank full of fish, and it can be tempting to purchase more fish than your tank can hold.

Calculating stocking density is an important step when setting up your tank. A long-used rule of thumb is one inch of fish per gallon of water. Consider the fish’s maximum size, not the size they are at the time of purchase. This means that a 20-gallon tank could hold 20 one-inch neon tetras or three 6-inch African cichlids – even though the cichlids are likely less than 2 inches when purchased.

While “the one-inch rule” is a good standard, always research the needs of the fish you plan to keep. Goldfish and other “thick” fish, for example, produce more waste than sleeker fish, and may therefore require two gallons per inch of fish.

Ensuring the right tank size is crucial, especially when caring for exotic freshwater fish with distinct habits. Take the case of a certain species of freshwater pufferfish, for example. Although it may only grow to be about 6 inches long, it’s recommended to provide these active swimmers with at least a 30-gallon tank. Why the need for such a spacious environment? Pufferfish are known for their curious nature and tendency to explore every nook and cranny of their habitat. A larger tank gives them the room they need to satisfy their inquisitive behavior and helps maintain their overall health and well-being.

Make sure that you perform adequate tank maintenance, especially if your tank is heavily stocked. Employ fish tank filtration appropriate to your tank size and perform regular water changes to maintain healthy water chemistry.

Providing Adequate Hiding Places

In nature, fish often seek refuge among rocks, plants, or other objects. You should include these in the aquarium environment to ensure aquatic animal welfare. 

Natural hiding places include live plants, stones, and driftwood. When using live plants in the fish habitat, you will need adequate aquarium lighting, and it is beneficial to do research on aquatic plant care. This can be a rewarding addition to your hobby.

If you’re not ready to tackle real plants, you can provide artificial hiding places. These include aquarium accessories such as artificial plants, driftwood, caves, hollow rocks, and whimsical decorations like Spongebob’s pineapple or the Roman Colosseum.

Whether constructing natural or artificial habitats, learn about fish biology and the specific needs of your preferred species. Many cichlids, for instance, prefer rocks to any other type of hiding place.

Feeding Strategies to Reduce Aggression

Fish nutrition is another important aspect of fish care. If your fish are especially aggressive at feeding times, try adjusting the frequency of feedings. You don’t necessarily need to feed your fish more food by volume. Instead, spread out the ideal amount over two or three daily feedings. This may reduce hunger and in turn, aggression.

You must also provide a balanced diet. Understand the marine biology of the fish you keep – are they carnivores, herbivores, or omnivores? For many species, you can acquire specific food blends. You might also supplement carnivore diets with protein-rich treats like tubifex worms or krill. Some herbivores enjoy a bit of lettuce, zucchini, or algae wafers.

Separating the Bully Fish

If a dominant fish or group of fish are harming others in the tank, you may need to move them to a separate tank. You can also obtain dividers for some tanks that can be used to temporarily or permanently subdivide the aquarium. This is especially helpful for dividing male and female fish for breeding purposes. 

Adding Compatible Tank Mates

Interspecies aggression is more likely to occur among “incompatible” species. Therefore, when setting up a tank or adding fish, you must choose appropriate species. Many pet and aquarium stores label fish “community,” “semi-aggressive,” and “aggressive.” Never put peaceful community fish with aggressive fish. Instead, choose fish from the same category.

While these categories are helpful, don’t forget to do your own research. Some fish may be incompatible even within the same category. For example, African cichlids are often kept in a dedicated tank not only because they are aggressive, but because they have specific water parameter needs.

Introducing new fish slowly is another effective strategy. You might place new fish in a breeder box for a time so that their tank makes can “get to know them.” Territorial fish are less likely to attack a familiar tankmate than one that has suddenly been dumped into their environment.

Use of Visual Barriers

If you must keep highly aggressive fish in the same tank, you may choose to use visual barriers to lessen confrontation. A highly planted or decorated tank will interrupt the bully’s line of sight – and if he can’t see his tankmate, he won’t chase him.

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