Acro-eating flatworms are the pest that makes people quit SPS. They are almost invisible on the coral, they eat tissue from the base up, and by the time you can see damage you usually have a population, not a passenger. The good news: they are very beatable, and the method is not complicated. It is just unforgiving of shortcuts.
How to know you have them
You will almost never spot the worm first. You spot the evidence:
- Pale or bleached patches starting at the base of an acro while the tips still look fine.
- Bite marks — small, irregular areas of stripped skeleton, often on the shaded underside of branches.
- Tiny white egg clusters laid in rows on bare skeleton near the base.
- A coral that will not extend polyps in one specific spot, day after day, with no other explanation.
Acro-eating flatworms are flat, oval, roughly 2–3 mm, and they take on the color of the coral they eat, which is exactly why staring at the colony rarely works. The reliable way to confirm is to remove the coral and dip it, then look at what falls off in the cup. That is the diagnostic step: the dip cup tells you the truth, the display tank does not.
The routine that actually clears them
One dip does not solve a flatworm problem, and this is where most people go wrong. Dips kill adult worms. Dips do not reliably kill eggs. The eggs hatch several days later and you are back where you started, convinced the treatment failed when really the schedule failed.
What works is a cycle:
- Dip the coral in an appropriate coral dip, agitating gently so worms are dislodged rather than just stunned.
- Blow the skeleton off with a turkey baster or pipette while it is in the dip — base, undersides, every crevice. This is the step people skip and it matters more than the dip itself. Physically removing eggs is what shortens the whole process.
- Rinse in clean tank-temperature saltwater before it goes back.
- Repeat every 5–7 days, at least three times. You are outrunning the hatch cycle, not winning a single fight.
If the affected coral is glued in place, this is the argument for keeping valuable SPS on removable plugs and frag racks. A colony you cannot lift is a colony you cannot treat.
Things that make it worse
Over-dipping. Doubling the dose or the time does not double the kill; it burns tissue and hands a stressed coral a second problem. Follow the times for the dip you are using.
Dipping in tank water and pouring it back. Never return dip water, rinse water, or anything that was in the cup to the display. That is how you reintroduce what you just removed.
Treating one coral. If one acro has them, inspect every acro. Flatworms move. Treat the tank as an outbreak, not an incident.
Assuming it worked. Check the dip cup each round. Fewer worms each time is success. Same number on round three means something in the routine is off.
Prevention is one habit
Dip everything, from everyone, every time — including corals from vendors you trust, including frags from friends, including that one piece that “looks clean.” Clean-looking is how flatworms travel. A dip takes a few minutes; a flatworm outbreak takes months.
Get the details right
Flatworms are one of about a dozen pests that hitchhike on frags, and several of them look similar enough to confuse and need different handling. Our Coral Pest ID & Treatment Guide is 29 pages of photos and treatments — flatworms, nudibranchs, red bugs, vermetids, aiptasia, asterina and the rest — so you can identify what is in your dip cup instead of guessing.
For the dipping routine itself, dose, timing, and a repeatable process, see The Complete Coral Dipping Guide, and Professor Polyp’s Bubble Bath Coral Dip is the dip we developed for exactly this work. Both are also in the Complete Reef Care Library.