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Flatworms refer to a variety of small (0.5mm - 10mm long), flattened, symmetrical turbellarian worms in tropical and coldwater aquaria often considered pests.


The term flatworms covers a wide variety of species with significantly different reactions to control so correct identification is important. For other types of worms see Nematode.

Convolutriloba (including Red Planaria?)

Convolutriloba retrogemma

Elongated body with rounded head and 3 lobes or projections on the tail end. Yellow/brown in color sometimes with a red gland near the tail end. These genera contain a toxin that is released when the animal dies. The animal will auto-lyse (self destruct) in response to stress. This may trigger other individuals nearby to auto-lyse as well. [1]. Identification of convolutriloba species.

Heterochaerus Australis

H. Australis

Elongated body with rounded head and 2 lobes or projections on the tail end. Yelllow/brown in color. Contains zooxanthellae. [1]


Transparent, large round head with 2 lobes or projections on the tail end. Considerd harmless [2]



Oval in shape with transparent bronze coloration. Contains symbiotic dinoflagellate. Not believed to prey on coral tissue [1].

Acropora Eating Flatworm (AEFW)

Oval, white. Aggresively consumes the tissue of Acropora sp. coral. Species unknown [3] [4].


In the wild: diatoms, algae, protozoan and copepods. In aquariums: small invertebrates including copepods, Artemia naupli, polychaetes and rotifers [5]. Some species contain symbiotic photosynthetic zooanthellae or dinoflagellates implying they derive at least part of their energy indirectly through photosynthesis.

Affect on aquaria

  • In large numbers, flatworms may cover coral starving them of light
  • The acropora eating flatworm is obviously a pest for acropora owners
  • Considered unsightly
  • Monitor your ammonia and nitrite levels as these flatworms tend to make an appearance on the sides of tanks as they attempt to escape the toxic water.


  • Manual removal. Repeated removal over several months can eradicate them [6]. Some techniques for removal:
    • can concentrate them in an area by shining a flashlight on a darkened tank for several hours
    • use powerhead and/or turkey baster to create high flow to dislodge from glass and rockwork, add filter sock/filter floss on drain to catch
    • siphon with airline tubing running into a bucket for disposal
    • siphon with airline tubing running to filter sock/filter floss in sump
    • turkey baster to suck out
    • paper towel to wipe off glass
  • Blackout tanks for several days. Some species contain symbiotic photosynthetic zooanthellae or dinoflagellates implying they derive at least part of their energy from light. Turn off all lights and block outside light sources for several days.
  • Reduce nutrients in the tank. Their diet isn't well known but it is likely either detritus or the higher level organisms that eat detritus. Some options:
    • Perform more frequent water changes
    • Vacuum the substrate
    • Feed less
    • Clean filters more often
  • Blue velvet nudibranch (Chelidonura varians) is known to consume flatworms. Like most nudibranches this species only consume one type of food (flatworms) and once the food supply is depleted they starve.
  • Several fish are reported to eat flatworms though the dietary preferences on individual fish will vary:
    • Freshwater:
    • Saltwater:
      • Six line wrasse (Pseudocheilinus hexataenia)
      • Yellow canary wrasse (Halichoeres chrysus)
      • Melanarus wrasse (Halichoeres melanarus)
      • Mixed reports of some species of dragonets and blennies
  • Salifert Flatworm eXit. A chemical treatment added to a tank targeting some species of flatworms.
    • This product is only effective against certain species. At 1.5x the recommended dose the moratality rates after 24 hours [7]:
      • Convolutrioba: 90%
      • Waminoa: 10%
      • H.. australis: 0%
    • Noted to be more effective at higher dosages but have a greater effect on other livestock
    • Noted to stun and sometime kill brittle stars, feather dusters, cerith snails and other delicate invertibrates.
    • The release of toxins by Convolutrioba is a serious threat to other livestock. Follow the manufacturer's instructions to mitigate.
    • 38% of aquarists in a survey lost livestock after using Flatworm Exit [7]. The source does not indicate if those 38% followed the recommended dosing protocol.
    • Multiple doses spread out across days or weeks is often recommended to ensure all offspring are killed.
  • Preventative measures when introducing new coral, live rock and substrate:
    • Shake in salt water to dislodge hitchhikers.
    • Manually remove visible flatworms.
    • Dip with CoralRx (effective against Convolutrioba, H. australis and Waminoa [7]).


External Links

Evaluation of Chemical Eradication Methods of Acoels (Acoelomorpha) From Marine Aquaria - Morphology of target species, Andrew H. Lynford M.S., Advanced Aquarist

See also