A second tank to rear the young and probably a good source of micro food for the fry. From everything I have read, it's really more trouble than it may be worth to rear fry from just one pair of clowns (and you really can only put one pair in a 55gal).
On the other hand, if you want to rear baby fish, get a freshwater planted tank and make sure you have some hornwort. Get a couple of Mollies (really, males are somewhat unnecessary as most mollies will come from the pet store pregnant already, or at least 4 of my six females did), and be ready for a baby boom.