good read :
One challenge you will find whilst keeping clown fish is that they tend to be very territorial in behaviour. It is not unusual for clown fish to peck each other. They are so territorial that they will fight any other clown fish that passes through their territory. Clown fish are all born males. The most aggressive of the clown fish will eventually turn into a female.
One way to deal with clown fish fighting is to have a larger aquarium to reduce the fight for territories. Planting marine plants in the aquarium can help by providing more cover and spaces. This will help, since they tend to stay away from each other’s spot.
When feeding the clown fish, it is important to spread the food evenly within the tank so that these clowns won’t have too much close interaction with each other whilst feeding. You can make matters worse if you do not give clown fish enough food, as this forces them to compete. They will recklessly fight and sometimes they can harm or even kill one another. Also bear in mind that food distribution matters more in an aquarium with an increasing population, since 3-month-old fish need more food than fish that are only a day old. Whilst it is true that a hungry man is an angry man, a hungry fish is also an angry fish!
It is important to keep the aquarium environment friendly. Clown fish may tend to be aggressive if they are under pressure or high stress. By maintaining optimum temperature levels and pH levels, you can reduce stress on the fish and avoid aggressiveness and fighting.
Sometimes if your clown fish population is one-on-one, they tend to fight each other more often. If you have 2 or 4 clowns in your tank, they each tend to single out one enemy and fight rigorously. Increasing the population a bit will help since the other clowns will act as detractors to the fighting clowns.
Clown fish may tend to become aggressive to one another during their breeding time. Males may fight for a female. Sometimes female clown fish may become very aggressive when they have laid eggs or have recently hatched eggs, as they seek to protect their young. But this typically passes as soon as the breeding time is over.
Have a great time clowning around with your clown fish!