Are popular subjects for the marine aquarium. They are extraordinary looking fish and so their attraction is obvious. But they have a down side - they are venomous. Those long flowing spines are hollow and at their base is a venom gland. The venom is purely for defence and not used aggressively or for catching prey it is designed to cause the maximum possible discomfort (pain) which is severe and will last for several hours. Placing the wound in very hot water will reduce the venoms effectiveness to some degree but be careful not to get carried away and burn yourself.
Pacu are normally very peaceful herbivorous fish. But they get very big and they have extremely powerful jaws and teeth which are easily capable of cracking open a brazil nut with a simple bite. A stray hand in their tank may arouse curiosity particularly if they are used to being hand fed and those teeth and jaws are easily capable of causing quite a serious injury. Possibly even removing a finger of an adult and almost certainly that of a young child. The only treatment for an attack will involve a trip to casualty and possibly surgery.
The wolf fish, Hoplias malabaricus is another fish which can cause a very serious injury with its powerful bite. These fish can be purchased when just a few inches long but they can grow to almost two feet in length at which time they need very careful handling.
They are also quite aggressive predatory fish and may attack any hand placed in their aquarium.
The foxface is another venomous species. The venom is injected through the spines of the fish's dorsal fin. The foxface is a very popular aquarium fish but it is less well known than Lionfish for being venomous. The venom is unlikely to cause serious harm to a healthy adult but it will cause a great deal of pain over several hours.
The difference between poisonous and venomous is this: Poison has to be ingested by the victim, where as venom is injected in to the victim.
Triggerfish have teeth and jaws for cracking open shellfish and biting off pieces of coral to get at the meat within. In the aquarium they are real characters and it is all to easy to feed them by hand especially as juveniles. But teaching them to associate your fingers with food is a great error because when they are a little larger a friendly inquisitive nip will cause a serious injury probably needing hospital treatment.
Of all the piranhas it is the black piranha which is often considered as being the most dangerous. They are reputed to have the most powerful bite of any living creature. Juvenile fish are usually quite timid but this timidness is replaced with aggression as the fish grows. Adults won't usually tolerate any tank mates and you should never place you hands in their aquarium.
The devil fish, Inimicus didactylusis extremely venomous and is closely related to the true stone fish. Apart from the venom it has the added danger that it is very difficult to see and it can easily go unnoticed.
They are often caught by prawn dredgers but they aren't a very popular fish in the hobby.