Beware of Poisonous Fish in Saltwater Aquariums

Red Lionfish (Pterois volitans)
Red Lionfish. Yiming Chen / Getty Images

Caution should be used when deciding if you want to keep a toxic saltwater fish. Some species not only have the ability to inflict a venomous sting that kills other animals, but many can cause toxic poisoning in an aquarium, which can result in the death of the other tank inhabitants, not to mention themselves. Fish such as these can also introduce a nasty poke or serious sting to humans as well!

A venomous sting that does not release toxins into the aquarium water will usually only affect the fish that was stung. This is often seen by a sudden and unexplainable loss of a fish when a stinging fish, such as a lionfish, is present. However, if all the fish in the tank are acting unusual, suddenly swimming erratically, or having the appearance of losing their sense of direction, there may be toxins in the water. The fish may show signs of heavy and rapid breathing, the eyes may cloud over, the fins become ragged, they lay on the bottom, and in the end, they will convulse and die. All of this can happen in a matter of minutes, depending on the toxicity of the poisonous fish or animal in the aquarium, and how large of an aquarium you have. Generally, the smaller the system, the faster the toxins can act.

If quick action is not taken, all can be lost. If it's not too late, immediately remove any living fish and animals from the tank. We heard that adding AmQuel to a contaminated aquarium helps to buffer and remove some of the effects of fish toxins. We tried this and have found good results. However, you still need to remove any remaining living animals from the aquarium until a complete and thorough tank/equipment cleaning and water change can be done. If the toxin from the poisonous fish is strong enough, sometimes it cannot be completely removed from the aquarium. If after having a toxic fish poisoning you have fish or other marine animals dying for no reason and everything else seems to check out, suspect residual toxins in the rocks, sand, gravel, carbon (if using for filtration), etc. This may warrant a complete strip-down and sterilization of the tank to rid yourself of the problem, but this is usually only in extreme cases.

Let's take a closer look at four common marine fish families that contain toxic fish that aquarists need to beware of:

Scorpionfishes (Family Scorpaenidae)

Fish in the Family Scorpaenidae are mostly marine fish that include many of the world's most venomous species. As the name suggests, scorpionfish have a type of "sting" in the form of sharp spines coated with venomous mucus. Their dorsal, pelvic and anal spines are able to inject toxin made by glands along the spine. The effects of the toxins of fish in this family are two-fold. They can inflict venomous stings AND cause toxic tank poisoning, both of which can kill other tank inhabitants. As far as human interaction, stings are usually not fatal (unless you are allergic to the toxin), but are extremely painful and can sometimes persist for months after the event occurs. Immediate steps can be taken to lessen the effects of a Scorpionfish sting, but time seems to be the key to recovery.

The Volitans Lionfish (Pterois volitans) is probably the most common Scorpionfish species kept by saltwater aquarists. The Hawaiian Turkeyfish (Pterois sphex) and the Hawaiian or Green/Dwarf Lionfish (Dendrochirus barberi) are two of the most common Scorpionfishes found in Hawaiian waters and are kept by aquarists as well. Lionfish stings are not fatal (unless you are allergic to the venom), but can be nasty to deal with. As far as the potency of their poison, no telling what it might to do your other tank inhabitants. I remember when I was working in a doctor's office (long before I knew anything about saltwater aquariums) and we had a tank in the waiting room with two beautiful Volitans Lionfish in it, along with some other nice marine critters. One day just before our lunch hour the maintenance guy came to clean the tank and got stung. He immediately took himself to the emergency room, as he was in extreme pain and his hand started to redden and swell up. When we returned from our lunch hour, EVERYTHING in the tank was dead!! No matter how hard we tried to re-establish the tank with new saltwater animals, everything just kept dying. The doctor finally got fed up, completely stripped the tank and started over with a freshwater tank, with no further problems after doing so. In regards to the maintenance guy, he was fine, but complete recovery from the injury took some time. Make note, it is wise to wear rubber gloves for protection when dealing with any potential stinging marine animal.

Many species of Scorpionfish are bottom dwellers and are masters at the art of camouflage. The stonefish waits in disguise as a rock on the bottom for prey to pass them by before attacking, pouncing on the prey and rapidly opening its mouth to create a suction that pulls the prey into the mouth.

Boxfishes (Family Ostraciidae)

Ostraciidae is a family of squared, bony fish belonging to the order Tetraodontiformes, closely related to the pufferfishes and filefishes. Ostraciid boxfish of the genus Lactophrys also secrete poisons from their skin into the surrounding water, further protecting them from predation.

When startled, frightened or harassed, the fish in this family have the ability to release a fatal toxin from their bodies. They can kill every living thing in your aquarium, including themselves, leaving you with just your decorations, rocks and tank standing. Use extreme caution when mixing these fish in an aquarium, unless you want to take the risk of possibly losing everything. If you desire to keep them, they are best kept in a non-aggressive species tank. Cowfish and trunkfish are also part of the Ostraciidae family and have this same characteristic trait.

Pufferfishes (Family Tetraodontidae)

The majority of pufferfish species are toxic and some are among the most poisonous vertebrates in the world. Also known as Balloon, Blow, Globe, and Toad fishes, these fish have the ability to store tetrodotoxin. In certain species, the internal organs, such as the liver, and sometimes their skin, contain tetrodotoxin and are highly toxic to most animals when eaten. Considered to be a delicacy by the Japanese, eating the meat of these fish, called Fugu, can result in severe food poisoning in humans, sometimes resulting in death, if not prepared properly.

Two of the most common types of puffers in Hawaii are the Green Whitepot/Stars & Stripes Puffer (Arothron hispidus) and the Brown Whitespot/Speckled Puffer (Arothron meleagris). Some of these fish have the ability to exude their toxins into the water, too, so caution should be used when mixing these fish in your aquarium. We have had the experience of putting a Brown Whitespot Puffer in a catch bucket with a few other fish while collecting, and when we brought the fish up from diving to take them home, everything in the bucket was dead.

We were told, years ago when we started our fish business, that the Spider-Eye Puffer (Canthigaster amboinensis) is one of the most toxic puffers in the Tetraodontidae Family. Caution should be used when adding these fish to aquariums, as they can exude a toxin into the water, in addition to being poisonous if eaten.

Tetraodontidae
Tobias Heinloth / Getty Images

Squirrelfishes (Family Holocentridae

The dorsal spines of Squirrelfishes have the capability to inflict a nasty poke that can result in something in comparison to a mild bee sting. For some people, this is merely a painful nuisance, but to others, it may cause more serious problems. Nothing to the point of resulting in death, just redness and swelling that may take time to heal.

In closing, while some of these fish families do not have the capability to exude their toxins, some do have parts of their bodies that can be poisonous if consumed by other fish. If left in a tank after death, other fish may eat the flesh and become sick or possibly die.

  • We cannot stress enough that being prepared for potential problems that may arise from owning any of these fish and that knowing all you can about them is of the utmost importance. To learn more about poisonous/venomous fish, as well as dangerous Octopuses, Nudibranchs/Sea Slugs and other marine animals, refer to scientific and verified resources.
Squirrelfish.
Humberto Ramirez / Getty Images