All clownfish are born without sex. They go from neutral to male to female, and they can not go in reverse. The most dominant clown in a group becomes the female and the second most dominant becomes the male. The rest will remain neutral. Usually when adding a second clown you want to go with one that is smaller than the one you have. This reduces aggression as the hierarchy is already established due to the sizes of the fish. The larger, more established clown in the aquarium will almost always become the female.