I think what everyone else has stated has been correct, clowns can be very aggressive towards each other during the domination period,but this doesn't always last long enough to harm either one if you get yourself a large female,One way which can help and which I have used(although not in such a small tank) is to seperate the male and female using a glass seperator for long enough to calm the female down as much as possible,if you get a female they can attack males for quite some time so patience is a must,once she seems calm(this could take up to 2 weeks)you can remove the glass,but this isn't fool proof