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Another Uninvited Visitor – Flatworms

When it comes to maintaining a healthy reef tank, many aquarists will agree to it that there are many intruders that are a threat. Some are disastrous in the long run while some cause instant debacle. Not all intruders are crooks and neither is every one of them friendly. While some worms are an absolute pro to reef growth and keeping the tank algae free, there are some, that have adverse effect and you better kick them out before they start bothering your marine creatures. One such intruder is the Flatworms. While some species are actually beneficial to reef tank there are two major species that bring in threat with them – brown rust flatworms and Acropora eating flatworms.

The obvious symptom is when you see red on your corals and they look to be increasing day by day. They are a real pain and you’ll know what I mean if you have encountered them. Brown rust flatworms are the most commonly found ones and can become a quick terror if left untreated. They are tan brown or rust color with small dots reaching up to 1/4″ in length. These are not predators of corals but the problem with them is that they grow furiously and can soon take over the aquarium. They can grow thick to the extent of blocking lights from reaching the corals. Without proper lights the corals are disabled from manufacturing food via photosynthesis and may become weak and stressed. Recent studies have proved that these flatworms battle with the corals for their planktonic food. On the whole, the brown rust flatworms will essentially feed on the zooxanthellae from coral tissue, thus damaging and can starve them to death.

Acropora eating flatworms are mostly popular in US and have recently been found affecting reef in Germany. These are unidentified species are cause more terror due to their aggressive nature. They are white and opaque in color, and are mostly oval in shape, making them very complicated to detect in the aquarium. They are usually noticed in damaged corals that are usually mistaken to be normal tissue damage. They are known to consume the actual tissue of Acropora sp. corals at a much rapid rate. They prefer species of Acropora that have shorter polyps, and are most commonly detected on Staghorn and Tricolor species. This flatworm is very persistent, and instant action should be taken go rid of them.

The place to spot Brown rust flatworms is on the top of the corals and usually in areas of low flow in the reef tanks. Acropora eating flatworms can be noticed in areas of massive tissue loss in colonies of Acropora. They can also be detected if there are gold to brown egg masses on the coral skeletons. It is necessary that you look into these signs as an alarm for immediate action in order to prevent them from spreading further. Delay can cause you to lose the corals.


The preventive precautions are the same; keep the tank in low nutrition, use of good protein skimmer and quarantine all new additions. Keep an eye on the skimmers to see if they are working properly. However, if you still get unlucky and these flatworms come in uninvited, you can take the following steps for treatment:

1.Manually you can remove the adults and eggs after transferring the effected corals in quarantine. Use tweezers or a toothpick to remove all of them and leave no trace behind. You can also siphon the flatworms off the coral using some small airline tubing through a syringe that can work in the same way. Keep them in quarantine for some days until you are assured that they ate worm free.

2.You can introduce some species of Wrasse like Six Line Wrasse, Yellow, Leopard Wrasse, and Dragonets such as the Spotted Mandarin into the tank that has infected corals. These are predators of these flatworms and will clear them off soon. They will also prevent them from coming back. The Blue Velvet Nudibranch is also a natural predator of these flatworms but they are sensitive to water changes and do not sustain longer.

3.Freshwater dip is also effective in removing flatworms. Since these flatworms are very sensitive to salinity changes, a 10 seconds dip in dechlorinated freshwater will make them lose their grip and fall to the base of the container. Make sure the freshwater has the same temperature and pH as the main tank’s water in order to lessen the stress level to the infected corals.

4.There are treatment chemicals available that will remove these flatworms no doubt but also kill the other useful worms. It is best if these can be chemicals are introduced in quarantine tanks. Once cleared keep them in for a few days before transferring to the main tank.