Bully Fish are fish which tease or attack other fish in your aquarium. Such fish can injure or kill other fish. Some may only chase other fish. However, fish which are chased can become stressful which may result in death.
Hence, it is necessary to identify and remove fish bullies from the aquarium.
Fish bullies can be kept in a separate tank.
Adding more plants and hiding plants will help smaller fish hide. This can sometimes solve the bullying problem.
Make sure that fish in you are adding are compatible with one another. Some fish are aggressive by nature. It is best that you avoid them.
Make sure that all your fish have enough to eat. Hungry fish may attack other fish or their slime coat. Ensure that all your fish in all levels of the aquarium have access to food.
If a particular species of fish is being attacked. Add more than one fish of the species to the tank. This will make the bully chase the fish in turn so that no single fish will be overly stressed.
Sometimes, mating behavior can be confused as bullying. Male guppies for instance will chase female guppies. Ensure that there is one more female than the number of males in the tank.
Fish which have laid eggs or have young can also become hostile.
Over crowding can also be a reason for bullying. Some fish are territorial. When their space is threatened, they can become aggressive. Ensure that your aquarium is properly stocked.
Some fish tend to get aggressive during the mating season.
If none of these things work, you can ask your pet shop if they will take back the bully fish. They may not return your money. Some pet shops may give you another fish in exchange.