Only Berghia spp. nudibranchs eat aiptasia, and they eat it exclusively - nothing else will keep them alive.
...which, all things considered, is how it works for most nudibranchs. They tend to be very, very specific feeders. Some eat only certain types of algae, sponge, or anemones, and some nudis have unknown diets. Basically, avoid the whole group if you don't know exactly what species you're buying and what it eats.