Spawning clowns get really aggressive. Whenever mine spawn, they even attack the mag float and my hand if I get anywhere near them. I guess they never heard of the expression "don't attack the hand that feeds you."
Depending on the clowns you get, you could probably get a flame or lemonpeel angelfish. Liveaquaria recommends min. 30 gallons. The only thing is, they're both hit and miss for nipping on corals.