Acropora eating flatworms (AEFW) are the pest that makes SPS keepers paranoid, and for good reason. They are nearly invisible, they match the colour of the coral, and they lay eggs that shrug off every dip on the market. The good news is that they are very beatable with a boring, disciplined routine.
How to tell you have them
- Bite marks. Small crescent-shaped marks on acropora tissue, often near the base or on the underside of branches.
- Tissue loss from the base up on one or two acros while everything else in the tank looks fine.
- Egg masses on bare skeleton: clusters of small brown or golden ovals. Finding these confirms it.
- The worms themselves in the bottom of a dip container. They are a few millimetres long, flat, and coral coloured.
Pull the most suspicious acro and dip it. The bottom of the container gives you the answer in a minute.
Which dip works
Adult AEFW are killed by most proper coral dips. Bubble Bath is labelled for flatworms at 1 part dip to 5 parts saltwater for 1 minute. Coral RX, Bayer-type insecticide dips and levamisole-based products like Flatworm Exit are all used against them too, and a 2021 study by Barton and colleagues found levamisole and praziquantel removed over 90 percent of adults without harming the coral.
The part that matters more than the brand: no dip reliably kills AEFW eggs. Eggs hatch in roughly a week to ten days, faster in warmer water. So a single dip, however strong, just resets the clock.
The schedule that clears them
- Dip every acropora in the system, not just the damaged one. Flatworms move between colonies.
- Baster the coral hard while it is in the dip, especially the base and underside, so dead and stunned worms come off.
- Remove eggs physically. Scrape egg masses off bare skeleton, or cut away the bare section. Remount small frags on new plugs.
- Rinse twice in clean saltwater and dump all dip water down the drain.
- Repeat every 5 to 7 days for 4 to 6 weeks. You are catching each hatch before it is old enough to lay its own eggs.
- Keep treated corals in quarantine if you can, so they are not re-exposed while you work.
Warmer water speeds up the egg cycle. If your tank runs warm, lean toward the shorter interval.
Prevention
AEFW arrive on acropora frags. Dip and inspect every acro that enters your system, check the plug and any bare skeleton for eggs, and swap the plug if in doubt. Quarantine new acros for a few weeks with a repeat dip at day 7. It feels slow. It is much faster than a six month outbreak.
Common questions
Can I treat AEFW in the display tank?
In-tank treatments exist, but most reefers get cleaner results by pulling acros and running the dip schedule, because the eggs on the skeleton are the real problem.
Do wrasses or other fish eat AEFW?
Some wrasses will pick at them, which helps keep numbers down. It is not a cure on its own.
Will dipping kill my copepods?
Dipping a coral kills the pods living on that piece, which is localized and expected. It does not crash the pod population in your tank.
Bubble Bath is sold at Coral's Coral. To identify AEFW and their egg masses next to the look-alikes, see the Coral Pest ID & Treatment Guide. For the general routine, read How to Dip Corals and our coral dip comparison.