A "red planaria" flatworm problem in a saltwater aquarium. Photo has been enlarged with a macro lens.

Photo is from the Reef2Reef archives.

Welcome to the second part and conclusion of the flatworm article. For Part 1, just go here, but I warn you, it’s a long one.

As I mentioned last time, flatworms are a perennial topic in our forums because some marine aquarists have problems with them. I wanted to devote an article (or two) to aquarium flatworms and how we (try to) deal with them. If you need some help with your flatworms, head over to the Reef Aquarium Discussion forum, and you’ll find plenty of people willing to help you.

For this article, because there are so many references, instead of having tons of links within the article, I have grouped all the references together at the bottom.

So, last time, I left you hanging with the suggestion that Flatworm eXit (by Salifert), the most popular “reef safe” remedy for a plague of flatworms, is actually the drug, tetramisole HCL, a close relative of levamisole HCL, and that Flatworm eXit (I think) is roughly 1/10 the strength of tetramisole HCL.

And I told you a story about an aquarist that had a severe reaction while using Flatworm eXit and having a bare arm in the tank. Well, if you do the math, which I won’t bore you with here, it turns out that using Flatworm eXit in your tank is an extremely weak solution of tetramisole HCL—if it is tetramisole HCL which is not proven.

It’s highly unlikely that Flatworm eXit caused this person’s medical emergency unless the person was allergic to the drug or had a very strong sensitivity to it—which is possible but unlikely. So, was this aquarist having a reaction to flatworm toxins released in the tank? Or allergic to something else in the tank, totally unrelated to this whole flatworm discussion?

We don’t know, and we’ll never know. But if you use Flatworm eXit in your tank, I mean you’d probably have to drink a gallon of your tank water to be at risk from the drug, and the salt would probably kill you before the tetramisole HCL would. (I’m half-joking. Please do not do this at home.)

Regarding flatworm toxins, released when flatworms are killed, we don’t know if that’s true for sure either. We do know that there are plentiful studies showing that some marine flatworms contain tetradotoxin.

Tetradotoxin is a sodium-channel blocker, a potent neurotoxin, that’s found in many marine animals including several of the fish belonging to the order, Tetraodontiformes. This toxin is probably best known as the toxin you’re trying to avoid if you eat “fugu”, an Asian dish prepared from fish containing this toxin. Chefs, who prepare fugu, are required to be licensed following a strict, long, and draconian training and apprenticeship.

A fish from order Tetraodontiformes.

Photo is a royalty-free image from Pixabay.

So, like I’ve said more than once—but it bears repeating—I don’t know if your flatworms contain toxins, but it’s definitely another good reason to wear gloves when you stick your hand in your tank, especially if you’re in the process of killing a ton of flatworms.

I don’t have a gas chromatograph at home or a laboratory or access to one. But if there are any biologists, analytical chemists, toxicologists, or other scientists reading this who would be interested in testing the aquarium saltwater following a treatment for flatworms, there are definitely plenty of inquiring minds who would love to know A) what Flatworm eXit is for sure and B) if there are any toxins released when flatworms die. Obviously, one test is anecdotal. A lot of tests on different aquariums would be super interesting.

It’s a notable coincidence that two products that do not tell you the ingredients, Salifert’s Flatworm eXit and Seachem’s Prime are both “reef safe.” Okay, I don’t believe in coincidences. In the saltwater aquarium business, there are very few products that are “reef safe,” and I think there is big money to be made on them. I understand the need for trade secrets and proprietary information. However, I have a problem with a company not willing to release this information in a medical emergency.

For DIYers, it’s possible to buy some of these antihelmintics as pure substances through many avenues: aquarium businesses, chemical companies, ebay, amazon, and through a veterinarian to name a few. Reef2Reef does not recommend buying chemicals through deepnet websites or from faraway countries where you can’t be confident that you’re receiving what you paid for. But if you’re going to try to dose pure chemicals yourself, then please be very careful to measure accurately—your aquarium livestock depends on it.

On another note, I asked a behavioral scientist I know, Dr. Rachel Grillot, to read these flatworm articles for her opinion on their readability before I published them. She immediately sent me a link to an article about a just-published study by a friend of hers about flatworm infections affecting the fecundity of women. If anyone doesn’t know what fecundity means, it’s fertility and ability to produce offspring.

Now this is a brand new study on a small population of women. We can’t possibly know if there are any conclusions to be drawn about other mammals much less fish. However, in closing, I’d like to mention that for those hoping someday to breed their marine fish, you might want to treat those fish for flatworm infections….or not.

A predatory *land* "hammerhead" flatworm, class Turbellaria and genus Bipalian.

Photo is a royalty-free image from freerangestock.com.

A special thanks is due today to the following individuals:

Tom Land, President Emeritus of the Washington D.C. Area Marine Aquarist Society, who spent a lot of time talking to me and discussing biochemistry. He is a treasure trove of information.

Rachel Grillot, Ph.D., who proofreads my articles for readability.


We encourage all our readers to join the Reef2Reef forum. It’s easy to register, free, and reefkeeping is much easier and more fun in a community of fellow aquarists. We pride ourselves on a warm and family-friendly forum where everyone is welcome. You will also find lots of contests and giveaways with our sponsors.


(1) https://en.wikipedia.org/wiki/Anthelmintic

(2) https://reference.medscape.com/drugs/anthelmintics

(3) https://www.reef2reef.com/threads/white-flatworms.85789/

(4) http://wetwebmedia.com/flatworms.htm

(5) https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1439-0469.2005.00301.x

(6) http://www.melevsreef.com/critter/benign-flatworm

(7) https://www.int-res.com/articles/meps/58/m058p041.pdf

(8) http://www.extreme-aquatics.com/707/saltwater/how-to-indentify-worms-in-your-reef-aquarium/

(9) http://wetwebmedia.com/WormPIX/FlatwormPIX/Flatworms2.htm

(10) http://wetwebmedia.com/flatworms.htm

(11) http://wetwebmedia.com/WormPIX/FlatwormPIX/Flatworms3.htm

(12) https://www.britannica.com/animal/flatworm

(13) https://biologywise.com/what-are-different-types-of-flatworms

(14) https://wamas.org/forums/topic/16160-dips-and-cures-red-bugs-to-acro-eating-flatworms/

(15) http://www.wetwebmedia.com/fltwmchemcont.htm

(16) http://www.melevsreef.com/articles/acropora-eating-flat-worms-aefw

(17) http://www.melevsreef.com/articles/how-to-eliminate-flatworms-red-planaria

(18) https://www.ccohs.ca/oshanswers/chemicals/ld50.html

(19) https://fscimage.fishersci.com/msds/32094.htm

(20) https://www.sciencedirect.com/topics/medicine-and-dentistry/levamisole

(21) https://www.jstage.jst.go.jp/article/suisan1932/52/6/52_6_1065/_article

(22) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5367013/

(23) https://www.sciencenewsforstudents.org/article/killer-flatworms-hunt-poison

(24) https://www.sciencedirect.com/topics/medicine-and-dentistry/anthelmintic

(25) https://reefs.com/magazine/pest-free-reefkeeping-iii-the-flatworms/

(26) http://www.salifert.com/?fbclid=IwAR3CynhuVQw61oeGRC0D-35uZzIUqm_Er-wxqvrnEcdHIFq0IG2DooMXvQs

(27) http://science.sciencemag.org/content/350/6263/970

(28) https://en.wikipedia.org/wiki/Tetrodotoxin

(29) https://pubchem.ncbi.nlm.nih.gov/compound/tetrodotoxin#section=Top

(30) https://www.bbc.com/news/av/magazine-18102811/preparing-fugu-japan-s-poisonous-fish-dish

Author Profile: Cynthia White

Cynthia received her BA in English from NYU during the Paleozoic Era. She has been a freelance writer and editor for over 20 years. She has written for newspapers and magazines, both in print and online and was formerly a marketing manager for a small oil company. Her portfolio can be found here. Now she is a writer and editor on staff at R2R, where her forum nickname is Seawitch. Her build thread can be found here.

For 15 years, she kept a dozen freshwater tanks, bred cichlids--Cyphotilapia frontosa--and sold them to pet stores in Calgary. Finally, after years of study, she has come to saltwater side. She lives in British Columbia, Canada, with her husband and three special-needs dogs, a five-minute walk from the Georgia Strait.