I agree with all above. An eel is an agressive creature, and will prey on many of the things that most people keep in a reef.
Also Lax mann is right, an eel is a critter that you need to carefully plan for, so if you want one dont buy it on the spot, do your research first.
It will require a secure top on the tank, and I mean SECURE, not even a 1/2" hole should be available anywhere to prevent it from jumping. Also it will need to be in an agressive tank, or it will likely eat all your small fish, shrimp, crabs, and various other critters. Also snowflake eels get pretty large quickly, so they are capable of even eating larger fish like tangs.