The acoel flatworm, commonly known as "Rust-Brown Flatworm" is a photosynthetic flatworm that can reach plague proportions. This is the main reason they are undesirable, looking unsightly and smothering low flow, light intense areas throughout the aquarium. The rust brown flatworm is a non predatary, photosynthetic flatworm, and as such, does not directly impact the inhabitants of the aquarium. Rust-brown flatworms tend to move slowly, but can cover a significant distance in a hour or so. By observing closely, can see them glide over the surface and wriggle from side to side.
One method of removal includes poisoning (via commonly available products such as Salifert's "Flatworm eXit"). This treatment can cause the flatworm to release toxins into the water column upon death, indirectly poisoning many of the tank inhabitants. It is therefore critical to remove as many flatworms as possible prior to treatment, by manual siphon using airline tubing during a water change. Implements could also include a syringe or turkey baster, although far more labor intensive.
Biological control is hit and miss. Fish such as the Banana Wrasse (Thalassoma lutescens), Sixline Wrasse (Pseudocheilinus hexataenia), and Mandarins (Synchiropus spp.) are all reported to eat the rust brown flatworms, but strong evidence is scarce, and accounts vary from total annihilation to completely ignoring the flatworm.
The flatworm cannot grasp onto objects strongly, and prefers a low flow and intensely lit location. They will quickly cover all surfaces void of heavy flow. For this reason, it is possible to use heavy flow to dislodge as many as possible for removal via skimming or mechanical filtration.
Manual removal, as stated before, using a siphon, can be efficient in removing large numbers periodically, only for them to return soon after.
- Aquarium flatworm under microscope - YouTube
- Chelidonura varians: the flatworm eating machine by Gregory Schiemer - Advanced Aquarist's Online Magazine
- Flatworms, including "Planaria" & Marine Aquariums - WetWebMedia