The lionfish is a very dangerous fish especially for divers. It’s found in the tropical Indian and Western Pacific Oceans, especially in reefs, where it hides during the day and hunts during the night. It can reach 40 centimeters and weigh up to 1200 grams; but what makes it part of this list of 10 most dangerous fish is that it has a poison gland with a very toxic substance. Contact with a human can cause inflammation, redness, bleeding, muscle aches, cardiovascular problems and breathing problems. Thorns of this species can cause a pain that lasts a few days depending on the amount of poison received.