Acoel Worms in Reef Aquariums: What They Are, Why They Bloom, and How to Control Them

What are Acoel worms?

Acoel worms, often referred to as “flatworms” or “planaria” in the hobby, are simple, gut-less invertebrates that glide across rock, glass, sand, and sometimes corals. They are not true planarians, but they behave similarly in reef tanks and can appear in a range of colors from pale tan and peach to reddish-brown. Two groups are especially common in aquariums: Convolutriloba, commonly called “red planaria,” which proliferate on sand and hard surfaces, and Waminoa, the peach-colored species that live on coral tissue. While most acoels are more nuisance than a coral parasite, unchecked populations can irritate corals, detract from a tank’s appearance, and during mass die-offs, release toxic compounds that stress fish and invertebrates. Understanding why they show up and how they multiply turns an anxious cleanup into a manageable, stepwise process.

What do Acoels look like and how to ID them?

Identifying acoels is straightforward once you know their look. Adults are usually 1–5 mm long, extremely thin (flat), and glide smoothly rather than darting or swimming. Colour is variable, but peach, tan, and reddish-brown are most common; Waminoa often look slightly opaque or speckled on coral surfaces. It’s important not to confuse acoels with polyclad flatworms. Polyclads are larger, often patterned, and are true predators that can eat snails, clams, and corals, an entirely different level of threat that calls for different tactics. If you are unsure, a macro photo with a scale reference (and where they were found—on glass, sand, or coral) helps confirm the ID.

How do Acoels get in my tank?

Acoels, like many other hitchhikers, can enter your aquarium on new corals, frag plugs, live rock, and macroalgae. They reproduce both sexually (laying eggs) and asexually (fragmentation/self cloning), which means a single missed individual can seed an entire population. Acoels like Convolutriloba species readily clone themselves by creating a new Acoel worm from tissue buds (located at the tail end). This bud quickly forms and within days is separated from the original Acoel worm to roam and grow to an adult. Even smaller individuals can bud and clone themselves creating even smaller Acoel worms. Overpopulation tend to occur in systems with high organic load, detritus, biofilms and bright lighting that favors microalgae and low-to-moderate flow that allows detritus to settle. In other words, acoels are most comfortable in the same pockets of the tank that collect “gunk.” Some species can also capture and feed on microplankton such as copepods on surfaces. Because some species harbor symbiotic algae (photosynthesis) and can graze on biofilms and mucus, they can persist even when traditional food sources seem scarce.

 

How Acoels feed and survive in your tank

Nutritionally, acoels are opportunistic mixotrophs (They eat, and they photosynthesize). On the food side, they can feed on coral mucus, biofilms, micro crustaceans like copepods and other surface organics that build up on rock, glass, and coral tissue. Many species carry symbiotic microalgae (Symbiodinium, Amphidinium and zoochlorellae) inside their tissues that provide Acoels with sugars and other photosynthesis products, just like in photosynthetic corals.  Photosynthesis alone can be sufficient to sustain populations under bright light (which is why you often see heavier blooms in high-PAR, high biofilm zones), and some species in the red-planaria complex are so reliant on their algae that they behave like obligate symbionts;  break the partnership and they “bleach” and crash. Some studies report that Convolutriloba can only survive total darkness for a few days to weeks (depending on the species), even if food and pray is available. Implying that light is more crucial for their survival than food or prey. This make is harder to eliminate by "starving" them, as turning the lights off in a reef tank for days or weeks is not an option.

 

How bad are Acoel worms and what damage could they cause in my tank?

In terms of impact, the picture ranges from cosmetic to consequential. Glass and rock covered in tiny teardrop-shaped worms is primarily an aesthetic problem and a sign that organics and surface films are abundant. Waminoa sitting on coral tissue is more serious: by layering over polyps, they can shade photosynthetic tissue, blunt polyp extension, and gradually sap the coral's vitality. The most acute risk arrives during mass mortalities, often after a chemical treatment when Convolutriloba in particular can release toxins as they die. If a large population crashes at once, fish may gasp, corals may retract, and the aquarium can take on a stressed look within minutes to hours. That is why any aggressive intervention must be paired with activated carbon, heavy skimming, and big water changes.

Aceol eradication strategy

Control starts with husbandry.  Its impractical to remove their food and energy source (biofilms, copepods and light) from your tank. It is best to begin with daily siphoning during the brightest part of your photoperiod, when acoels are most visible and concentrated. Running a length of airline tubing into a filter sock in the sump lets you remove as many as you can without throwing away large volumes of water. At the same time, increase flow to eliminate detritus pockets/zones, rinse or replace mechanical filters more frequently, and control the feeding so that dissolved organics are reduced. Reducing peak light intensity or photoperiod for a week can starve photosymbiotic species and slow reproduction but it will impact your corals too.

For corals with Waminoa, targeted dips are an efficient, coral-safe way to knock populations back. Iodine-based and broad-spectrum coral dips both work when used exactly as directed, followed by a clean saltwater rinse and return to the tank. Because eggs often survive dips, repeat the dip every three to four days for two to three rounds. For colonies fixed to large rocks, regular turkey-baster blasts paired with strong crossflow can keep worms from accumulating.

Biological control- Acoel predators

Introducing natural predators is a good way to reduce and control their population. Biological controls are best viewed as supporting actors rather than silver bullets. Several Halichoeres wrasses, the yellow “coris” wrasse (H. chrysus) and the melanurus wrasse (H. melanurus) commonly feed at acoels and can reduce their population over time, especially in tanks with open sand and rockwork. Sixline wrasses (Pseudocheilinus hexataenia) sometimes help but can become territorial in smaller systems. Dragonets (Synchiropus spp.) were reported eat aceol worms opportunistically. If you use biological control, add them with realistic expectations: they can significantly reduce the population but rarely eradicate the problem alone.

 

Chemical treatments 

When Aceol worm populations remain high despite proper tank maintanance, manual removal and biological control, a chemical treatment can be justified. It should be done taking extra precautions as some species (mainly Convolutriloba) will release toxins as they die.  The released toxins can negatively impact some corals invertebrate an fish.

Products like Flatworm eXit contain Levamisole Hydrochloride as the main active ingredient. Levamisole is an anthelminthic (anti-parasitic/de-wormer drug) primarily used for treating roundworm and hookworm infections. Levamisole is an acetylcholine receptor agonist causing continued muscle contraction leading to paralysis and death

potential negative effects of such treatment is sometimes mistaken to be due to the treatment itself, but it is mainly due to the toxins released from the dying aceols.  Therefore, before chemical treatment, siphon as many worms as possible to reduce the number of aceols that will die in your tank. Have activated carbon on hand (roughly 100–200 g per 100 L is a reasonable starting point) and fresh saltwater ready for a 25–50% water change. With all pumps running, dose a reputable flatworm treatment exactly per label instructions and monitor the tank closely for the next hour. As worms release or die, continue siphoning them out. 

Once you see a clear decline in living numbers, add in the fresh carbon in a high flow area, perform the large water change, increase skimming, and replace mechanical filtration if dead worms were caught in filter socks. Because potential in your tank are not affected and will hatch later, plan a second treatment five to seven days afterward, again with siphoning any visible aceols, carbon, and a large water change. This two treatments approach is what prevents a population return.

 

Prevention is the cheapest, least stressful solution and comes down to quarantine and clean handling. New corals should be dipped on arrival, observed in a separate system for two to four weeks, and re-dipped mid-quarantine to catch late hatchers. Frag plugs and discs benefit from physical cleaning or re-mounting to fresh bases. Dedicated tools and buckets for quarantine and display prevent accidental transfers, and a routine of steady nutrient export and sensible feeding denies established acoels the conditions they need to explode. Even in well-run systems, you may see a few worms from time to time; that alone is not cause for alarm. Act when numbers trend upward or when you see worms lingering on coral tissue.

In summary, acoel flatworms are opportunists that prosper in well lit aquariums where food films collect and flow is gentle. Most do not consume coral tissue directly, but they can stress colonies, spoil the look of a tank, and, in large numbers, create toxin risks during die-offs. The winning strategy is layered: maintenance to cut their fuel, siphon methodically to reduce the population, use dips for coral-level problems, lean on selective predators for ongoing population pressure, and reserve chemical treatments for infestations that are not controlled by the former methods. Follow this and you’ll turn an infestation into a temporary maintenance task.