i read somewhere that for the first day or two they feed off of their sacs they hatch out of, but i got this information off of a website that tells how to raise the fry.
www.breedclownfish.com
Your fry tank should constantly be full of rotifers for the clownfish fry to eat. In the days leading up to the hatch you should harvest some of your rotifers and place them in the fry tank. I usually put a small amount of rotifer diet in the tank to make sure they also have something to eat. You do not want to turn the water as green as your rotifer bucket, but some food will keep them alive until the fry are in the tank. You might notice some symptoms of your clownfish dying. A struggling clownfish will have trouble swimming. Sometimes you will see them just spinning in the water column. Other times, they will often sink towards the bottom and then suddenly start swimming again. If you see this happening, your clownfish are starving to death. Add rotifers as soon as possible. You will be able to see your fry eating the rotifers by noticing them swimming, then stopping, curving their tail and then darting forward. This is a good sign that the clownfish are eating the food you have provided. Continue to feed the fry rotifers for about a week. Around day 5 I usually get some live brine shrimp ready. Frozen brine shrimp will not work and will only will foul the tank.