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Venomous Tankmates – Just Be Careful

Posted on Monday, November 16th, 2009 at 4:47 pm by

As a marine aquarist, we all seek a common goal. That goal is to fill out tanks with the most beautiful and visually appealing marine life that we can get our hands on. I know hat I personally love nothing more than the rush of a potentially venomous sting from one of my beloved pets. I get it. We are all gluttons for reef-punishment. However, with proper precaution and a well thought out stocking strategy you can successfully house a reef filled with the deadly gems of the sea. This article is going to outline the fish-on-fish violence vs. the fish-on-human stings that we will all indubitably encounter with time.

Many fish are marine fish are poisonous, but some carry a deadlier sting than others. There are some fish that are only able to inflict a venomous sting upon other fish and then there are some fish that can release toxic poisons into the entire aquarium. I refer to these fish as “fuckers”. Now that you are aware of the two different attack styles it is time to identify their symptoms. If a single fish has been stung and a large amount of poison has not been released into the aquarium, you will generally only be alerted by the sudden death of that fish. If a healthy fish suddenly dies, there is a strong chance that lurking somewhere within the tank is a predator. The indications of the second type of venomous attack are much more apparent and detrimental to experience. If a poisonous fish has released a fair amount of toxins into the aquarium the fish will begin to swim erratically, potentially loose their sense of direction, breathe in a rapid manner, lay on the bottom of the aquarium, or have a cloudy appearance in their eyes. All of which, will generally lead to a dramatic passing including a convulsion period and ending in death. All of this can happen rather quickly, and depending on the size of the aquarium and the toxicity of the fish your personal experiences may vary.

There are several families of commonly found poisonous fish in the home aquarium. They, along with a few of their venomous characteristics are mentioned below. Although limited, the information should help guide you through any future stocking predicaments that you may encounter.

Box Fish & Cowfish

These fish have a hard, rectangular shaped body, thus giving it its name “Box” Fish. They also have firm plate-like hexagonal scales. It uses a small dorsal and anal fin to propel itself through the water, at which time it curls up its tail on one side or other of the body. It swims in a rowing manner. The mouth region protrudes from the front of the body, looking similar to a snout with a small mouth at the end.

Toxic Profile
This little fish might look harmless, but when frightened it will release a toxic poison from their skin to protect itself. However, in a closed system, the boxfish can kill everything including itself. This is totally bad news bears.

Additional Precautions
Because of its nature for being toxic, use extreme caution when having one in an aquarium. They get along fine with other fishes, are very non-aggressive, but be sure you don’t mix other fish that will harass them. It is not wise to put two Box Fish of the same sex in a tank together, They will fight right off the bat. This is Dynasty-Style drama that no reef aquarium needs.


The spines of the dorsal fin and the rays of the pectoral fins are unusual, because they are very long and extend far beyond the membranes connecting them. All of the pectoral rays are unbranched and the upper pectoral rays, in particular, are developed into long, feeler-like filaments. These fish are absolutely stunning and adored by many hobbyist, but be weary of anything that can send you running to the E.R. (if a human is stung, the recommended treatment is to immerse the appendage into near boiling water until the pain subsides… OUCH! I say ask the doctor about that before giving it a go)

Toxic Profile
This fish is as painfully venomous as it appears. Extreme caution is highly advised when keeping any members of the lionfish family. They are venomous to other fish and humans.

Additional Precautions
This fish is traditionally a bottom dweller that likes plenty of sheltered hiding places. They will also prey upon many of your aquarium cleaners and smaller fish. Although the fish is not generally aggressive, they do have a tendency to eat fellow tank-mates.


Highly compressed body with stout pectoral fins used for “walking” and support. They are found in a wide variety of colors such as red, pink, brown, green, orange, purple, rust, black, and undoubtedly others. These fish also have a true jaw teeth. Not to be overly redundant with my wording, but OUCH! Teeth and venomous spines on one fish!

Toxic Profile
Their spines are venomous stinging tools and therefore should be kept and handled with immense caution and care. Their spines are covered with a venomous mucus that will protect it from any potential danger or serve to attack their loyal caregiver… You.

Additional Precautions
Because it will most often only eat “live foods” in captivity, it does not adapt well to aquarium life. I would recommend doing a ton of research before diving into the Scorpionfish family.


These fish are primarily identified by their scaleless bodies, ability to inflate. and round edged fins. It has the ability to inflate by inhaling air or water. This is a protective defense that Puffers have which prevents them from being eaten by other fish. When it expands and inflates itself, a predator finds it difficult to swallow or get its mouth around the fish. When inflated, this Puffer’s body has a soft prickly texture, which is harmless to the touch, yet intriguingly attractive and fun to view.

Toxic Profile
This fish will release venomous toxins through it’s skin when threatened causing the second type of aquarium poisoning. Also when this fish dies there is a strong possibility of it releasing poison into the system killing off your other aquaria. In addition, to having a toxic attack method, many puffers are known as “nippers”, meaning they are able to use their teeth to nip at their fellow tank-mates.

Additional Precautions
This fish should be kept with caution. They will nip at your corals, pick off your cleaning crustaceans, and do a bit of damage to any fish that might be easily bullied. They also, depending on species are able to grow to large sizes. Do adequate research before selecting one of these brilliantly entertaining fish.

Sea Cucumbers

They typically have thick, muscular bodies equipped with tube feet used for clinging to the substrate and moving about, and pointed fleshy projections called, papillae often cover the body. Sea Cucumbers come in many varieties and many beautiful color combinations, making them an appealing choice for aquarists.

Toxic Profile
Sea Cucumbers expel poisonous toxins into an aquarium when stressed or after death. Due to the many different toxic natures of Sea Cucumbers it is important that you do research on the specific cucumber you are considering placing within your tank.

Additional Precautions
Sea Cucumbers can grow to exceptionally large lengths. In this case, size does matter, so choose wisely. Also, make sure that your tank will be able to support the dietary needs of the Sea Cucumber.


The body forms of Nudibranchs vary enormously. Nudibranchs often have venomous appendages located on their sides which look like some form of exotic orchid. They are as deadly as they are beautiful. Often brightly colored and natural flatworm predators they are selected for the home aquaria without much research. Do the homework. It is better than going in blindly and having sudden mass casualty.

Toxic Profile
Nudibranchs have a trait of excreting a toxic mucus when disturbed. This poisonous excretion can foul up the water and cause a rather quick biological crash that can be deadly to all other reef habitants.

Additional Precautions
Nudibranchs are sometimes marketed as superb algae eaters, but this is false information since all known Nudibranch species are carnivorous. I’m just saying they like a little meat in thier diet and for long term success you will need a well stocked aquarium. Always research your particular Nudibranch species to find out what type of diet it needs. Good luck with these gorgeously toxic critters.

I hope that you found this information on venomous aquaria useful. As always, and clearly overly mentioned throughout this article please be diligent in your own research and be your reef’s best advocate. Good luck and great reefing!

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2 Responses to “Venomous Tankmates – Just Be Careful”

  1. brandon klaus says:

    poisonous and venomous are different. poison is consumed while venom is injected (via spines, fangs, etc). and you forgot urchins and rabbitfish/foxface.

    • RT says:

      Excellent points B, I’ve contacted the author and will make the change.

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