Ringing your local Poison Center, and immersing the area of entry site with water as hot as you can tolerate are immediately called for. Found in tropical Pacific and Red Sea rocky reefs, ten to two hundred feet of depth. They should not be hiding in a corner at the top or bottom. Dittos for hard, labored or accelerated breathing, off-color or red patches and torn fin membranes. A sure beauty mark that indicates a good individual is the presence and shape of the supraorbital antennae.
Lionfishes evidently consider themselves top reef dogs.
Best time to net them, right out of the open water, is between sun ups and downs. The usual malarkey here for permits and capture and transport paraphernalia. Lionfishes, for all their apparent slow-moving, calm breathing, seemingly low metabolic lifestyles need space . Though lions don't appreciate fast swings in temperature, they have enormous range tolerance. I have 'found' them in forgotten tanks at wholesalers in incredibly saline water, so much they should have been floating on top.