Just make sure the clowns are comfortable. That means a sleep number mattress, soft music, and the thermostat set to 70. Kidding. I've found that if clowns have a nem that's hosting them, they tend to breed. I'm assuming it's because their comfortable. That said, you don't need a nem, since clowns use many things as a host, including flower pots, which many people use in their broodstock tanks.