There are some species that it is best not to feed by hand like eels, and fish that have large teeth. When you feed by hand they begin to associate your hand with food which is great until you need to put your hand in the tank to do some maintenance. Now since you don't have some food in your hand they may decide to sample your hand instead. Not a big deal with a clown fish, but if you had say a large trigger or a Hawaiian dragon eel it probably isn't going to end well.