An important part of reef tank ownership is unfortunately preventing and eradicating pests and parasites. This includes flatworms, which are a nasty pest as they can become so numerous that they block out light that corals’ tissues need or begin to consume the coral tissues themselves. They will also eat the micro fauna living on rocks. The biggest issues arise if they get into water pumps or other equipment, because if they get crushed they will release a deadly toxin that has the potential to kill corals and fish if the concentration is high enough. Most aquarists will at one time or another get an infestation of flatworms. Thankfully due to advancement in technology it is easier than ever to remove these pests from your beloved aquarium.  

Types of Flatworm

Firstly, let’s look at identifying whether you have flatworms. There are two distinct types of flatworms, also known as Planaria, that are fairly common. 

Rust Brown Flatworm (Convolutriloba retrogemma)

This species is brown to rust colored, with a red dot, reaching up to around ¼” in size. They are a slightly elongated oval shape, with two tail-like appendages at their posterior. The Rust Brown Flatworm is the most common type of flatworm and can quickly become a problem as they will proliferate rapidly in aquariums with high nutrient levels. 

Acropora-eating Flatworm (Amakusaplana acroporae)

The prevalence of the Acropora Flatworm is increasing, which is concerning as this is a more aggressive and invasive flatworm species. The Acropora Flatworm can eat the Acropora sp. Corals’ tissue at a rapid rate. To make matters worse, this species is difficult to spot in your reef aquarium as it is white to opaque in coloration. It has been noted by expert aquarists that this flatworm prefers Acropora with shorter polyps, such as Staghorn types and Tricolor species. 


As always, when dealing with pests and parasites, prevention is better than cure. By maintaining low nutrient levels this will help to reduce the risk of an outbreak of these pests. Low nutrient levels can be maintained through the use of carbon and aggressive protein skimming, as well as increased water flow. For new specimens, ensure these are properly quarantined before adding them to your reef aquarium. 

  • Natural control methods can be effective at controlling populations but may not feed on all of the different species of flatworm or completely eradicate the problem. Species that can be used to naturally flatworms include: 
    • Wrasses species including the Six Line Wrasse (Pseudocheilinus hexataenia), Yellow Wrasse (Halichoeres chrysus), or Leopard Wrasse (Macropharyngodon sp.);
    • Dragonets such as the Spotted Mandarin (Synchiropus picturatus); or 
    • The Blue Velvet Nudibranch (Chelidonura variants).
  • As flatworms are very sensitive to salinity changes, another suitable option to remove flatworms is to perform a brief freshwater bath or dip. This is achieved by dipping the colony in dechlorinated freshwater for 5 to 10 seconds and then shaking the colony while submerged in the bath of freshwater. This will cause the flatworms to lose their grip and fall to the bottom of the container. It is important to ensure the freshwater is the same temperature and pH as the aquarium’s water to reduce the amount of stress on the colony.
  • Manual removal is an effective although time-consuming option. Many expert aquarists have found siphoning to work the best, as the flatworms do not keep a very strong grip on the colony. To siphon the flatworms out, you will need some airline tubing, 3/16” plastic tubing, and a media filter bag (airline tubing elbows are also optional). Turn off all your circulation pumps and place the rigid tube into the aquarium and start it siphoning. You can place your filter bag in your sump or a bucket in front of the tank. Using airline tubing limits the amount of water that is sucked out unnecessarily whilst you locate the flatworms. For delicate mushrooms and large polyp stony corals, small syringes work very well. It is possible to get the syringe very close to the flatworm and gently draw the worm into the syringe without harming the coral.
  • Finally, you can use a chemical treatment. This is an extremely effective method of killing the flatworms, however a word of caution. The chemical treatment itself is safe for your corals, fish and invertebrates. However, when the flatworms die they will release a toxin that is a huge bioload on your tank, which will kill everything else in your tank. If you use a chemical treatment, you will need to syphon the flatworms out quickly whilst using a lot of activated carbon to absorb any of the offloaded toxins produced by the worms.

With severe infestations, you may need to employ a combination of the methods mentioned above in order to eliminate or even control the populations of these pests over a number of days.