Gloves, especially when handling any Zooanthid or Polyp. Not much eats these anumals and there's a really good reason why.
If a rash quickly develops, the area swells rapidly, or you experience breathing difficulty or a racing heart rate call 911 immediately.
Venom injections from fish should immediately be treated with hot water - as hot as you can stand and then just a bit hotter directly on the wound.
Bristle worm stings:
If it's burning or a lot of pain treat with hot water first.
Many bristles are Calcium carbonate. Submerging the area in pure vinegar will dissolve them, even under the skin.
The few that are silica are more difficult.
Shake off any water but don't touch the area. Apply a coating of elmers glue and wait for it to completely dry. Carefully peel off the glue and most of the bristles should come off with the glue. Next, apply duct tape. The new clear Duct tape is especially effective. rub it on real good then slowly peel it off.
Urchins:
If it's burning or there's a lot of pain, treat it like a fish sting with hot water. You really need to remove any imbedded spines, they are a big problem. If you can't get at them go to the emergency room to have them removed.
Bang