Mastou is right, the prettiest ones (chromodorids and other dorids), and most/all of the others, in fact, are almost impossible to keep in a reef tank. They either eat up the suitable food and then die of starvation or, more likely, they start starving to death immediately after you introduce them, since it's highly improbable that you're growing the specific sponges or other food sources that the nudibranchs will need. Some may even release toxins into your tank when they die or are stressed. They are beautiful, but you should probably resist the temptation to buy these animals.
Cheers,
Don