if you don't want to breed them then feed once a day or once every couple of days. If you feed them 2 or 3 times a day they will come into breeding condition and start to prepare a nesting site and become highly territorial. However, clownfish are pretty placid even when guarding a territory, (by comparison to most damselfish) and in a reasonable size tank most species won't cause any real problems to the other tank mates.
I feed mine on marine mix, which is a combination of fresh prawn, fish, squid, spinach and various other marine organisms mixed in. Raw prawn is also good for bringing them into breeding condition.