Flatworms that eat coral in reef tanks

Brown Flatworm, Turbellaria, Kanula, Susac Island, Adriatic Sea, Croatia

It was originally assumed that keeping multiple corals healthy in a reef tank was difficult for the typical aquarist. Corals may now be successfully kept in the home aquarium thanks to advances in science and technology over the last decade. Soft corals, such as Mushroom corals, were among the first to produce good results in home aquariums, owing to their lower lighting requirements. Many small polyp stony (SPS) and large polyp stony (LPS) corals were added to the list of successfully kept corals as aquarium lighting improved and knowledge of what illumination corals require grew.

Because of the vast quantity and retained by hobbyists, "fragging" or fragmenting corals soon became a popular method for mass-producing corals. "Frag exchanges" proliferated like yard sales, and the number of corals migrating from one tank to another exploded. Sadly, the number of parasites and other pests that damage corals has also increased quickly. The Rust Brown Flatworm and the Acropora Eating Flatworm are two of the most common unwelcome hitchhikers that harm corals.

Rust Brown Flatworm

The most common flatworm seen in home marine aquariums is the Rust Brown Flatworm (Convolutriloba retrogemma). It's a rust brown to tan hue with a brilliant red dot approximately three-quarters of the way down its body, and it'll grow to be about 1/4" long. They have two tail-like appendages and are oval and elongated. In nutrient-rich marine aquariums, it reproduces quickly. When present in large numbers on a coral's surface, these flatworms can prevent proper light from reaching the coral, thus starving it. Some speculate that this flatworm eats the zooxanthellae that live on the coral's surface. This flatworm can be spotted crawling on the surface of the corals in sections of the aquarium with little water flow.

Acropora-Eating Flatworm

This flatworm (Amakusaplana acroporae) is oval in shape and white to opaque in color. It rapidly eats the real tissue of Acropora corals. Smaller polyped corals, such as the Tricolor and Staghorn species, appear to be its favorites. Rapid tissue loss on Acropora specimens and the emergence of gold to brown egg masses left on coral skeletons indicate the presence of this flatworm.

Controlling Flatworms

Both the Rust Brown Flatworm and the Acropora-Eating Flatworm can overrun the corals in your tank if left unchecked. There are several methods used to control flatworms in your tank.


It's a good idea to quarantine new coral specimens the same way you would a new fish. While in quarantine, the new species may be thoroughly examined for flatworm infestation and treated to eradicate any flatworms that may be present before being released into your display tank.

Mechanical Removal

The Rust Brown Flatworm, unlike the Acropora-Eating Flatworm, does not attach itself to the coral and may be readily removed with a tiny (1/4" airline) siphon. Simply start a siphon and gently suck the flatworms off the corals' surface, taking careful not to touch the corals' surface with the tube.

A quick freshwater plunge or bath is another option. Simply shake the coral after submerging it in a container of dechlorinated freshwater for 5 to 10 seconds. Flatworms are extremely sensitive to variations in salinity, and they will quickly lose their hold on the coral and sink to the bottom of the container. Balance the salinity and pH of the coral before immersing it to match the saltwater it originated from.

Natural Method

The Sixline Wrasse (Pseudocheilinus hexataenia), the Yellow Wrasse, and the Spotted Mandarin are all natural predators of flatworms. The most significant disadvantage of using this approach to clear your tank of flatworms is that the fish will not swallow every flatworm in the tank. Flatworm consumption is thought to be great in the Blue Velvet Nudibranch (Chelidonura varians). The biggest issue with the 2-inch Blue Velvet is that flatworms are its sole source of nutrition, and after it has gotten rid of all the flatworms in your tank, it will starve to death.

Chemical Treatment

Chemical flatworm solutions are available on the market that appear to function effectively without damaging your other tank inhabitants. Flatworm ExitTM by Salifert is a commonly used flatworm eradicator with a lot of positive feedback.

If you use a chemical treatment, make sure to remove all of the dead flatworms from the tank, since they may contain poisons that will be released back into the tank water as they decompose.

The best therapy for flatworms is obviously prevention. The least invasive way would be to quarantine young corals after giving them a freshwater bath. If a detailed check reveals the presence of flatworms while in quarantine, treating only the affected coral will have the least impact on your display tank.