The Rust Brown Flatworm (Convolutriloba retrogemma) is the most common flatworm found in home aquariums. ... It 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, effectively starving the coral.

Will flatworms harm corals?

The populations of these pests can increase to a point where they will actually perch on corals, and block the light from reaching the corals' tissue. Some experts believe that the flatworms actually feed on the zooxanthellae from coral tissue, thus damaging the corals themselves.

Are flatworms bad for reef tank?

The problem flatworms taking over your aquarium.

They may irritate some corals. Populations of flatworms like this in a closed system usually wax and wane, so if you can be patient, then they may very well recede on their own in a very weeks or months.