TrekLightly posted a photo:

Spotted this one while snorkeling near Koh Talu Island in Thailand.

Sometime called a devil firefish, the lionfish grows up to 35 cm (14 in) in length. The dorsal fin has 13 long, strong spines and nine to 11 soft rays, and the anal fin has three long spines and six to seven soft rays. The dorsal fin appears feathery and the pectoral fins are wing-like with separate broad, smooth rays. These fish vary in color from reddish to tan or grey and have numerous thin, dark, vertical bars on their head and body.

The common lionfish is mainly nocturnal and may hide in crevices during the daytime. It feeds on fish and small crustaceans. It has few predators, probably because of its venomous spines, but larger lionfish do prey on smaller ones.

The spines are highly toxic and have proved fatal to humans.
(Wiki)