Red flatworm (pest) (Convolutriloba retrogemma) care guide

Group

Other invertebrates

Other names

Red planaria, Rust-brown flatworm, Red bug planaria, Acoel flatworm

Family

Convolutidae

Origin

Originally Cuba; now found in aquaria and reefs worldwide

Water

saltwater

Temperament

peaceful

Reef safe

no

Care

The red flatworm, often called 'red planaria' in the hobby, is a reef aquarium pest and should not be sold as livestock. It is not a true planarian or flatworm but an acoel, a simple worm in the group Xenacoelomorpha. It is a tiny reddish-brown animal about 2 mm long (sometimes larger), flat and oval with a notched, lobed rear end. How it arrives: on live rock, coral frags, macroalgae and in water from other tanks. A few unnoticed worms can become thousands within weeks because it reproduces both sexually and by budding and splitting. How to identify it: look for small rust-red ovals gliding over rock, sand and glass and gathering in brightly lit spots and on coral tissue, often with a red film. It carries symbiotic algae, so it seeks light. Do not confuse it with harmless copepods or with sacoglossan slugs. Harm done: in large numbers it shades corals, smothers polyps and stops them opening, and spoils the look of the tank. Like other acoels it contains toxins; when many die at once they can poison corals and fish. Removal and prevention: quarantine and dip new corals and inspect rock before adding it. In the tank, siphon worms out during water changes, reduce excess nutrients and feeding, and use natural predators such as some wrasses (six-line, yellow coris) and the blue velvet nudibranch, Chelidonura varians. Commercial flatworm treatments work quickly, but you must be ready with a large water change and fresh activated carbon straight after, because dying worms release toxins. Copper-based medicines and some plant fertilisers are toxic to shrimp, snails and most invertebrates; do not use copper against flatworms in a reef. Sources: Wikipedia (en.wikipedia.org/wiki/Convolutrilobaretrogemma). Checked October 2026.

Diet

Gets much of its energy from the symbiotic algae in its body, so it thrives under strong reef lighting. It also eats small invertebrates such as copepods and rotifers.

Breeding

Reproduces rapidly, both sexually by laying eggs and asexually by budding, splitting in two and fragmenting, which is why infestations explode. There is no reason to breed it.

Compatibility and tank mates

Harmful in reef tanks: plagues of these worms smother corals and can release toxins that harm corals and fish. Eaten by some wrasses, mandarin-type fish (inconsistently) and Chelidonura varians nudibranchs.

Last updated 8 October 2026. Community-edited care guide on Aquatic-Auctions.