Mild sting? lol
Don't put your hand in the water!!!!
Seriously. I have a pretty good idea what those tiny little specks in suspension in that tank are.
I was on a 3rd grade class field trip in the Keys and my entire class was annhiliated by one of these. Two of them hospitalized overnight, everyone came back with massive welts. (Except for me, I was in the boat for "bad behavior", stayed too long with my snorkel under the water a few minutes before, and a chaperone thought I had drowned)
Granted, knowing what I know now about invert stings, it may be possible that a class full of kids who each have experience swimming in the Keys would have a stronger reaction to the stings than average people. Sting exposure probably makes one more vulnerable to stings from this creature, in the same way that handling anemones makes you more vulnerable to stings in a home aquarium.
Great video link by the way!