A marine flatworm magnified 60X.


Photo is from the Reef2Reef archives.

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.

All flatworms belong to the phylum, Platyhelminthe, and everything under Platyhelminthe is a flatworm, although some of these flatworms are better known by other names like flukes or tapeworms. Among the flatworms, there are lots of differences between types of flatworms. Some are free-living, some are parasitic, some are found in freshwater, and some are found in marine environments, for example, just to name a few of the differences.

Acoelomorpha is a subphylum of Platyhelminthe, and sometimes in marine aquarium discussion, you will hear people say, “that’s an acoel worm, not a XYZ flatworm.” Also, what we commonly call the Red Planaria flatworm is usually a species within the genus Convulotriloba (which happens to be under the subphylum Acoelomorpha).

Taxonists usually divide the more than 20,000 members of Playhelminthe into roughly four (4) classes: Cestoda (including tapeworms), Trematoda (including flukes), Turbellaria (including planarians), and Monogenea, although the members of Monogenea are often debated, and Monogenea might be actually a subclass under Trematoda. And obviously classification information changes all the time as new research is published and new species are discovered. (12) (13)

And that’s all the time I want to spend on flatworm classification and vocabulary other than to add that drugs or chemicals for controlling or killing flatworms are called antihelmintics. Because we can’t do this all day, you know. (1) (2)

Moving right along, there are typically four (4) types of flatworms that the marine aquarist will run into, and these types have zero to do with the classification by taxonomists. These four types can be loosely and arbitrarily described below:

1. The benign types of small flatworms. By this I mean the small (like just few millimeters) often translucent ones that don’t seem to bother any other livestock and don’t reproduce to crazy numbers in your closed system. Some of these are in the family, Convolutidae (under subphylum Acoelomorpha), but not necessarily all of them. (3) (4) (5) (6) (7)

2. The benign types of larger flatworms. The second type would be a larger type of flatworm that often arrives as a hitchhiker on live rock. You may never see this flatworm, except that you might notice that occasionally a small snail disappears or dies with only the shell left behind. These flatworms might be an inch long or bigger, might have bright colors, unusual patterns, and might have ruffled edges resembling a nudibranch. However, if you raise tridacna clams, I would be more concerned about seeing flatworms because some are predatory and can bother your clams. (8) (9) (10) (11)

3. The flatworms that are a problem for reef aquarists. I’m going to group together any flatworms that are a problem. By “problem” I mean flatworms that reproduce like mad and take over the aquarium. There are too many types to list here, but a common one is the one usually referred to as Red Planaria. (25)

4. The Acropora-Eating Flatworms or AEFW. These are a serious problem if you’re trying to raise acropora (SPS) corals. They are hard to get rid of and hard to even see to diagnose the problem in the first place.

So, let’s talk about these four categories one by one and what to do or not do about them.

1. Small benign flatworms. There’s probably nothing to do. These tiny flatworms are not bothering anything. Some people have a “no flatworm in my aquarium” policy, but that’s not really necessary. If the flatworms aren’t reproducing like crazy, they probably aren’t a problem, so do nothing unless you’ve got clams, then you have to be vigilant.

2. The larger benign flatworms. Probably not necessary to do anything, and you might only have one. They probably aren’t bothering anything. If you’re very worried, you could pull one out and destroy it. Some flatworms are predatory toward clams. See #1 above.

3. The problem flatworms taking over your aquarium. You may or may not want to do something about these flatworms. They are unsightly, but not necessarily bothering anything except your pride. They may irritate some corals. Populations of flatworms like this in a closed system usually wax and wane, so if you can be patient, then they may very well recede on their own in a very weeks or months. If you are determined to get rid of them, then there are few things you can do, which I’ll cover last.

4. The dreaded AEFW flatworm. This is a serious problem for acropora corals. The only way that I’ve come across to deal with this is to treat the corals away from the display tank. That means removing all affected corals and treating them with either povidone iodine or Betadine. I found many references to using three (3) ml of Betadine per liter, but this is anecdotal, and you will have to do your own research here and take your chances. I also found some sources saying that Betadine is just a brand name and different concentration of povidone iodine. I found other sources saying that the chemistry and molecular structure of povidone iodine and Betadine are slightly different. So, I can’t tell you what works best. This is uncharted territory. I also read about some aquarists treating corals with one of several antihelmintic medicines. Of course, a better solution (no pun intended) is a coral bath before you have a problem and before any coral goes in your tank, but that advice is not helpful if you already have the coral in the display tank with AEFWs eating them. And I’m not being flip. (14) (15) (16)

An AEFW flatworm problem.

Photo is from the Reef2Reef archives.

So, now let’s go back and talk about #3, the problem of a bazillion tiny flatworms in your tank.

What can you do about it?

1. Do nothing. There’s a good chance they will disappear on their own. Vacuum out as many as you can get to as often as you are motivated to do so, and keep your water column in excellent shape, and wait. And wait some more.

2. Biological controls. There are a couple of things you can try, but there are no guarantees. Some fish will eat flatworms, in theory anyway. I found references to several different wrasses including the yellow wrasse, the six-line wrasse, and the melanurus wrasse as possibilities. (The melanurus is usually touted as the best bet.) A few references suggested one or more types of mandarins. The success may depend on the personality of the individual fish or how hungry he or she is or the species of your particular flatworm. And it’s not so easy for home aquarists to identify their flatworm species. But this method of control is chancy at best. Another potential problem here is do you want that particular fish in your tank permanently? Does that fish go with your other livestock now and/or in the future?

Another possible biological control would be a nudibranch. There are some nudibranchs that will eat flatworms. Some will eat only flatworms; they are obligate flatworm eaters. The one most often seen in the aquarium trade is black with blue stripes, called a blue velvet nudibranch. Get one of these fellows, and they will (probably) eat all your flatworms. But then you have a new problem. After a few days or weeks, once the nudibranch has cleaned up your tank, it has nothing else to eat and will die of starvation. Some people would view this as cruel. One possibility would be to have an arrangement with the LFS to return or give back the nudibranch once it has cleaned your tank. It’s something to think about.

A blue velvet nudibranch.

Photo is from the Reef2Reef archives.

3. Chemical controls. And last but not least, there are chemical control possibilities. The product best known for treatment of a flatworm plague is Flatworm eXit by Salifert. This product is considered “reef safe,” in that you can use it in your display tank without harming too many other inhabitants. Maybe some other worms or feather dusters will suffer. At least that’s what the package says.

But hold on. Using Flatworm eXit is not a “fix it and forget it” solution. It’s a commitment. It’s a lot of work. Why? Because when a massive amount of flatworms die at the same time in your aquarium, something happens, and the water becomes very toxic. Why? It’s not clear. Some say that flatworms give off a toxin when they die. Others say, no, it’s not a toxin, it’s just a massive spike in nitrite and/or nitrate and/or ammonia and/or phosphate, depending on who you ask. Whatever the cause is, if you don’t move fast, you’ll have disposed of all your flatworms, and then the rest of your tank can crash in quick succession.

Interestingly enough, I did find references to toxicity in marine flatworms. Serious toxicity. Now I’m not suggesting that the flatworms in your aquarium are highly toxic, but I want to at least mention the possibility because it is well documented that some marine flatworms are highly toxic. (21) (22) (23) (24) (25)

Does tetradotoxin ring a bell? We’ll talk about that in Part 2.

So, how do you use the product safely? Read the instructions. What’s important is to remove as many flatworms as possible by siphon before the treatment and during the treatment. Have new saltwater mixed and aged and heated and ready to go. Have plenty of carbon ready to turn on to help absorb whatever is in the water. Marc Levenson of melevsreef gives a detailed explanation of how he successfully used Flatworm eXit. (17)

The Flatworm eXit label
Screen Shot 2018-11-20 at 2.32.06 PM.png

Screenshot courtesy of Cynthia White, 2018

Some people recommend trying Flatworm eXit on a few flatworms in a bucket of water to determine the dose you need to kill them before starting the treatment. As long as you know the exact volume of your test water in the bucket, you can multiply that up to what you’ll need for your tank.

What exactly is Flatworm eXit? We don’t know. The product ingredients aren’t labeled. Many people think it’s levamisole hydrochloride. Many people think it is not praziquantel, another antihelmintic routinely used in baths for incoming fish. Some people suggest it’s fenbendazole.

But Tom Land doesn’t think it’s any of those drugs.

Tom Land is the President Emeritus of the Washington D.C. Area Marine Aquarist Society, (WAMAS), and I talked to him on the phone on Monday of this week. Many years ago, an aquarist friend of Tom’s suffered a severe reaction while using Flatworm eXit (and having a bare arm in the tank.) At that time, Tom knew that a Salifert executive had an identity on one of the aquarium discussion forums, so, Tom contacted him.

Tom explained to the executive what was happening and asked for the exact ingredients of Flatworm eXit. And the Salifert executive would not give Tom the information or the Material Safety Data Sheet (MSDS). The only information he would offer to Tom was the LD-50 information from the MSDS, and he suggested to Tom that perhaps the aquarist was having a reaction to flatworm toxins and not to Flatworm eXit. That is certainly possible, although I did find plenty of references to patients having problems and reactions to antihelmintics. (24)

Before we talk about this some more, let me just say that the afflicted aquarist did fully recover and did return to reefkeeping.

Now if anyone doesn’t know what “LD-50” refers to, it means the “lethal dose” for 50% of test subjects. So, if you’re looking up a medicine or chemical to figure out how lethal it is, like on a Saturday night when you have nothing better to do, you can usually find out the LD-50 for mice or rats or dogs or whatever. (18)

The LD-50 that Tom was told from the Salifert executive for Flatworm eXit was the following:

“The presence of an anthelmintic (dewormer) with an oral LD50 of 4800mg/kg (rat) and 2100 mg/kg (mouse) and the presence of citric acid in the fomulation.”

Tom didn’t know then—and doesn’t know today—if the Salifert executive was telling him the truth. However, Tom did a little detective work and found that those LD-50 numbers match the MSDS for tetramisole HCL, another antihelmintic that is closely related to levamisole HCL except that the numbers for tetramisole HCL are 10X the dose that exists in Flatworm eXit or rather Flatworm eXit is 1/10 the strength of tetramisole (perhaps diluted to 1/10 the strength with the addition of citric acid). Tetramisole HCL is apparently a racemic form of levamisole with equal parts left- and right-hand isomers, while levamisole is only the left-hand form. (19) (20)

I have much more to tell you on this exciting topic, but I must stop here. Tune in to Part 2 for the rest of the story about flatworms, toxicity, and Flatworm eXit.

NB: I attempted to talk to Salifert before publishing this. I was unable to contact them through their website, and they did not reply to my inquiry through their Facebook page several days ago. (26)

A special thanks is due today to 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.


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(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

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.