The following was taken from this article:
http://www.tmc-ltd.co.uk/fish-invert...s/lionfish.asp
"Lionfish will always try to consume fish and invertebrates which are smaller than themselves - even fish that look too big may be found to disappear! The best way to avoid loss of livestock, is to keep a Lionfish-only aquarium, and to make sure,when choosing a group of Lionfish, that they are all of a similar size. "
Also, Lions aren't always reef safe, so if you're got corals or inverts in your tank, they may get eaten/picked at as well.