Flatworm on Corals - How to Deal with Them?

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Sooner or later almost every owner of a marine aquarium encounters them. When buying a new coral or exchanging corals with a friend, we run the risk of transferring this parasite to the aquarium. If we already have flatworms in the tank, then after several weeks there’ll be much more of the unwanted guests.

In this article I will describe what flatworms are and how to get rid of them. The main of these unwanted hitchhikers that affect corals are Rust Brown Flatworm and Acropora Eating Flatworm.

Rdzawobrązowy płaziniec/Rust Brown Flatworm

Rust Brown Flatworm (Convolutriloba retrogemma)

This species is brown to rusty in colour, with a red dot. It is the most common representative of flatworms found in domestic marine aquariums. They have a slightly elongated oval shape and two caudal outgrowths at their posterior end.

In nutrient-rich waters in marine aquariums, they reproduce very quickly. By encrusting the coral, this parasite can easily block light access to the coral.

Płazińce zjadające akropora

Acropora Eating Flatworm

This white or transparent flatworm is oval in shape. It is capable of eating a large portion of the coral tissue in rapid succession. The presence of this flatworm can be detected by the loss of acropora tissue and by the pink eggs on the coral skeleton.

Combating flatworms

There are several methods to combat flatworms. When you buy new corals it is essential to inspect them carefully and dip them properly.

  • Mechanical removal
    Unlike Acropora Eating Flatworm, Rusty Brown flatworms do not attach to corals and are easily removed with a small hose. Simply aspirate the pest with the hose.
  • Dipping
    Simply use a suitable dip for corals. Follow the instructions on the product.
    You can also make the dip in RO water. Submerge the coral for 5-10 seconds by moving it in the water. Flatworms are very sensitive to changes in salinity and in a short time will lose their adhesion to the coral and sink to the bottom of the container. 
  • Animals helping with the pest
    Marbled mandarin, Pseudocheilinus hexataenia and other wrasses – these fish will control the flatworm population.
    Blue Velvet Nudibranch (Chelidonura varians) is a food specialist that eats flatworms. Unfortunately, after the Nudibranch gets rid of all the flatworms, it will no longer be able to survive in the tank, as this is the only type of food it feeds on.  
  • Chemical preparations
    There are several preparations on the market that can help control the pest. One of them is Flatworm Exit. When using the treatment, keep in mind that you will have to suck off the dead flatworms. They are toxic and should be vacuumed out so that they don’t return anything to the water.


As you know, it is definitely better to prevent than to cure, so it is easier to dip corals and clean them from eggs before placing them in the aquarium than to act later on a living organisms in the aquarium.

About the author

Picture of Marek Protasewicz

Marek Protasewicz

Reefkeeping has been my passion for over 10 years now. I love learning. The hobby has taught me many valuable lessons, patience being the best example. Combining work and passion is my path. I run Crazy Coral, a marine aquarium shop, for a number of years. Building this business from the scratch I learnt from my own mistakes at a heavy cost.
Later I managed a project aimed at development of methods for quick growth of Corals in non-natural conditions. The project was carried out by Get Sales, Poland. Presently, I am responsible for distribution strategy at Reef Factory, of which I am a co-founder. The company produces smart devices for marine aquaristics. The last projects I have been involved in are Social Reef and ReefPedia.

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