Acoel Flatworms (Red Planaria): Reef ID & Control

Identify rust-brown acoel flatworms, understand why a population crash can poison the tank, and use the safe siphon-first removal sequence before any commercial flatworm treatment.

Overview & Identification

The flatworm most often called 'red planaria' in the reef hobby is Convolutriloba retrogemma, an acoel flatworm. They are tiny, about one-eighth of an inch long, and reddish- or rust-brown in colour. In bright tanks they appear as a spreading rust-coloured film over rock, sand and the bases of corals.

They reproduce rapidly, either sexually by laying eggs or asexually by fission or budding, where the worm splits and produces a bud that forms a new worm. This is why they can bloom into dense sheets within weeks.

Where They Come From

Acoel flatworms arrive on live rock, corals and frags, then bloom in nutrient-rich tanks where copepods and rotifers are abundant. They feed on these small invertebrates rather than on coral tissue itself.

Harmful or Beneficial?

These flatworms do not eat coral; they consume small invertebrates such as copepods and rotifers. The danger is indirect: in large numbers they shade and smother corals, and a mass die-off releases toxins that can cause significant damage in a reef tank.

Control & Removal

1. Manually siphon out 75-85% of the total infestation before dosing any product.
2. Add natural predators: six line and leopard wrasses are popular, and the blue velvet nudibranch (Chelidonura varians) solely preys on Convolutriloba species and mows them down.
3. When dosing a commercial flatworm-removal treatment, follow the manufacturer's instructions exactly: filtration is normally switched off for the treatment (no skimming, carbon, GFO or biopellets) while circulation pumps keep running, and worms typically begin dying within about 15 minutes.
4. Run fresh carbon and do water changes after the worms die to strip released toxins from the water.

Prevention

Control nutrients so blooms cannot explode, and dip or quarantine new corals and frags before adding them. Keeping a resident wrasse helps suppress small populations before they reach crash-risk densities.

Common Mistakes

Dosing any flatworm treatment into a heavy infestation without siphoning first is the classic error: the simultaneous die-off releases enough toxin to crash the tank. That risk comes from the mass die-off of the worms themselves, not from any particular preparation. Leaving filtration running during dosing can also remove the medication before it works.