According to Marine Fishes by Scott Michael, you could keep one in a 30 gallon. I like this book, but sometimes I don't understand how he decides on minimum tank size. This fish could grow to 6.7 inches according to the book, and in that case, I definitely wouldn't recomend a 30 gallon tank. I don't know much about lionfish, so I'm just throwing that out there, I'd wait for some other opinions.