What you're seeing is the clowns "fighting" for dominance. The clowns have a very weird social/mating habit. They are all born male and the fighting is actually determining which one becomes female (the female will become slightly larger than the male, hence the small one "twiching" and the bigger one nipping at it). In a group they have one female and the rest are all male, should the female die, get eaten, leave ect. The whole process starts over again. Soon after the "fighting" stops you will notice they spend a lot of time close together. You now have a mating pair of clowns and will see spawing as soon as they find a suitable place to lay their eggs. Then dad hangs around the eggs to protect them (he won't kill anything just chases fish or even your hand away). It is a slightly difficult and time consuming job to raise the fry though. Some do it, I don't. Its your call. Congrats!!