* Competition for resources: If the fish are competing for food, space, or other resources, they may become aggressive towards each other. This is especially true if the tank is overcrowded or if the fish are not getting enough food.
* Territorial behavior: Some fish are territorial and may become aggressive towards other fish that they perceive as a threat to their territory. This is especially true for fish that live in groups or schools.
* Predation: Some fish are predators and may prey on other fish that are smaller or weaker than they are. This is especially true for fish that have sharp teeth or claws.
* Disease: Fish that are sick or diseased may become more aggressive than usual. This is because they are under stress and may be more likely to lash out at other fish.
* Incompatibility: Some fish species are simply not compatible with each other. This may be due to differences in size, temperament, or behavior. If fish are incompatible, they may become aggressive towards each other.
It is important to do research before adding fish to a tank to make sure that they will all be compatible with each other. If you are not sure whether or not two fish species are compatible, it is best to err on the side of caution and keep them separate.