Lionfish (Pterois volitans) are unusual looking marine fish that belong to the family Scorpaenidae. There are other species of lionfish and they all have similar characteristics, but each species varies in size and appearance. Even members of the common lionfish species (Pterois volitans) varies in coloration and markings. All members of the family Scorpaenidae have venomous spines that they use to defend themselves when threatened.
When they feel threatened, lionfish lunge forward with their venomous spines erect to attack whatever it is they feel threatened by. This includes aquarists who put their hands into the tank to clean, rearrange items, or catch a fish. If you get stung by one of these fish you should seek medical attention immediately. However, most people do make a full recovery after being stung by lionfish. There are other members of the family Scorpaenidae that are more venomous than the lionfish, but hopefully you won't find these for sale at your local petstore.
Lionfish reach an adult size of 15 inches or 38 cm in length. Because they are large fish they need to be housed in a large aquarium (at least 55 gallons). You should provide plenty of hiding spaces.
You must be careful when choosing tankmates for lionfish. They are predatory fish that will eat smaller fish and will also eat the inhabitants of a reef tank. It can be kept with members of its own species, but I'd make sure they were of similar size.
Lionfish are carnivores and will eat meaty frozen foods (marine fish and crustaceans) that have been thawed. If at first your lionfish refuses to eat, it can be acclimated to eating in captivity by feeding it small feeder shrimp or fish (it seems mean, but this is what lionfish eat in their natural habitat). It is from the Indo-Pacific.
You can typically purchase lionfish from between 40-100 dollars depending on the size. Smaller fish are less expensive.