SALTWATER disease

Polyclad (turbellarian) flatworms (Convolutriloba spp., Waminoa spp.)

Reviewed by the Fast Aquatics husbandry team · Updated May 2026

Quick referencePolyclad/turbellarian flatworms (Convolutriloba retrogemma "rust brown" and Waminoa) coat reef rock + corals in flat brown patches. Smother coral; treat with Flatworm Exit + manual siphoning.

Symptoms

Brown flat worms 2-5mm on rocks
Brown patches on coral
Coral tissue recession
Worms visible on glass

Cause

Convolutriloba retrogemma + Waminoa flatworms hitchhike on coral + live rock. Reproduce rapidly - one worm becomes thousands in 2-4 weeks. Common entry point: SPS frags, zoanthid colonies.

Treatment options

Salifert Flatworm Exit. Standard treatment. CRITICAL: siphon ALL visible worms first. Dying flatworms release toxins; mass die-off can kill the tank.
Manual siphon. Powerful siphon hose into a 5-gallon bucket. Suck up worms before treatment. Repeat daily.
Six-line wrasse (predator). Pseudocheilinus hexataenia eats flatworms. Caveat: aggressive to other wrasses, may not survive QT.
Coral dip (Bayer). For new frags: dip in Bayer Advanced (imidacloprid) per manufacturer. Kills hitchhiker flatworms.

Always treat in a separate quarantine or hospital tank. Most medications are toxic to coral, invertebrates, and live rock biology. Consult an aquatic veterinarian for valuable fish.

Prevention

ALWAYS dip new corals in Bayer Advanced or CoralRx 5-15 minutes. Quarantine corals 4 weeks if possible. Siphon any visible flatworms immediately on sighting.

Fatality + outcome

Low directly. High indirectly - mass die-off after Flatworm Exit can crash a tank if siphoning is skipped.

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Aquarium-keeping fundamentals

Whatever specific topic brought you here, four fundamentals govern long-term aquarium success: water quality, parameter stability, biological filtration, and species-appropriate husbandry. Skip any one and the others struggle to compensate.

Water quality: ammonia + nitrite at zero, nitrate under 30 ppm freshwater + 10 ppm reef. Test weekly with API or Salifert kits. Use our water parameter checker to score your readings against your tank type.

Parameter stability: stable wrong parameters beat fluctuating ideal parameters. Most fish tolerate a wide pH range if it's stable. Sudden swings of 0.4+ pH or 5+°F kill fish faster than chronic suboptimal values. Use temperature controllers (Inkbird) + automated dosing for consistency.

Biological filtration: the bacterial colony on your filter media + rock + substrate is the engine. Never replace all media at once. Use our filter turnover calculator to size correctly.

Species-appropriate husbandry: research adult size, territoriality, diet, and tankmate compatibility before purchase. Use our tank stocking calculator + compatibility guides.

Frequently asked questions

How long does an aquarium take to set up? 4-6 weeks for full cycling + first stocking. Use our cycle ETA calculator + how long does cycling take.

What's the best aquarium for beginners? 20-gallon long. Big enough for parameter stability, small enough for budget + space. See beginner picks.

How often should I do water changes? 25-30% weekly. See water change frequency Q&A + water change calculator.

Why does my fish keep dying? 5 leading causes: uncycled tank, wrong species pairings, no quarantine, undersized tank, neglected water-change schedule. See full diagnosis.