Coral Eating Flatworms in Reef Tanks

Rust Brown Flatworm and Acropora-Eating Flatworm

Brown Flatworm, Turbellaria, Kanula, Susac Island, Adriatic Sea, Croatia
Borut Furlan/WaterFrame/Getty Images

Flatworms in Reef Tanks

It used to be thought that it was almost impossible for the average aquarist with a reef tank to keep many corals in a healthy state. With the advancements in both the science and technology over the past decade, it is now possible to keep corals in the home aquarium successfully. Due largely to their lower lighting requirements, soft corals, such as Mushrooms were among the first corals to yield good results in the home aquarium.

As aquarium lighting improved and the understanding of what corals require, many SPS and LPS corals were added to the list of successfully kept corals.

As a result of the number and variety of corals being kept by hobbyists, "fragging" or fragmenting corals quickly became a popular method for reproducing corals on a fairly large scale. "Frag swaps" sprang up like yard sales and the number of corals moving from one aquarium to another grew rapidly. Unfortunately, the number of parasites and other pests that destroy corals grew even more rapidly. Chief among these unwanted hitchhikers that affect corals are the Rust Brown Flatworm and the Acropora Eating Flatworm.

Rust Brown Flatworm

The Rust Brown Flatworm is the most common flatworm found in home aquariums. It is rust, brown or tan in color with a red dot and will reach a size of about 1/4". They are oval and somewhat elongated with two tail-like appendages.

It has been identified as Convolutriloba retrogemma and reproduces rapidly in nutrient rich marine aquariums. In high concentrations on a coral's surface, these flatworms can actually keep adequate light from reaching the corals, thereby "starving" the coral. Some believe that this flatworm also consumes the resident zooxanthellae on the coral's surface.

This flatworm is normally found in areas of the aquarium with low water movement and can be seen on the surface of the corals.

Acropora-Eating Flatworm

This flatworm is white to opaque in color and oval in shape. It consumes the actual tissue of Acropora sp. corals at a rapid rate. It seems to prefer the smaller polyped corals, such as the Tricolor and Staghorn species. This flatworm's presence can be detected by the rapid loss of tissue on Acropora specimens and the appearance of gold to brown egg masses left on the coral skeletons.

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.


When bringing new coral specimens home, it is wise to quarantine them as you would a new fish. While in quarantine, the new specimens can be closely inspected for flatworm infestation and can also be easily treated to eliminate whatever flatworms are present before introduction to your display tank.

Mechanical Removal

Unlike the Acropora-Eating Flatworm, the Rust Brown Flatworm does not attach itself to the coral and is easily removed by using a small (1/4" air line) siphon.

Simply just start a siphon, and then gently vacuum the flatworms from the surface of the corals, being careful not to contact the coral's surface with the tube.

Another method is to use a brief freshwater dip or bath. Simply submerge the coral in a container of dechlorinated freshwater for 5 to 10 seconds and shake the coral. Flatworms are very sensitive to changes in salinity and will lose their grip on the coral and fall to the bottom of the container in a short period of time. Before dipping the coral, balance the salinity and pH to match that of the saltwater the coral came from.

Natural Method

Flatworms have a number of natural predators, including the Sixline Wrasse (Pseudocheilinus hexataenia), the Yellow Wrasse and the Spotted Mandarin. The biggest drawback to ridding your tank of flatworms with this method is that the fish will not consume every flatworm in the tank.

The Blue Velvet Nudibranch (Chelidonura varians) is believed to do an excellent job of consuming flatworms. The greatest problem with the little (2") Blue Velvet is that flatworms are its entire diet and once it has rid your tank of all the flatworms, it will slowly starve to death.

Chemical Treatment

There are a number of chemical flatworm products on the market which seem to work fairly well without harming your other tank occupants. Salifert’s Flatworm Exit™ is a widely-used flatworm eradicator and has several good reports.

If you elect to use a chemical treatment, be sure to siphon all of the dead flatworms out of the tank, as they may contain toxins which can be released back into the tank water as the flatworms decay.


Obviously, the best treatment for flatworms is prevention. Quarantining new corals after giving them a freshwater bath would be the least intrusive method. While in quarantine, if a close inspection shows flatworm presence, treating only the infected coral will lessen any impact on your display tank.