Most lionfish that you see in the store are the volitans lionfish also known as a turkey fish or common lionfish or spotless lionfish or russel's lion fish. This fish is cool looking, however, he grows up to 15 inches and needs a tanks size of 55 gallons minimum. There are a few lion fish that can fit in your size tank that are also cool. They are: clearfin lionfish, deepwater lion fish, spot fin lionfish, zebra lionfish, short fin lionfish, and twin spot lionfish.
Hope this helps.