Flatworms are one of those “wait… what is that?” reef tank surprises. The good news is that many flatworms are manageable if you catch them early and respond safely. The key is correct ID—because “flatworm” can mean a mostly harmless nuisance, or a coral-eating pest that needs a more serious plan.
Below is a beginner-safe, step-by-step approach for flatworms in a reef aquarium, including when to treat, how to avoid toxin issues during die-off, and how to prevent them long-term.
What “flatworms” are (and why reef keepers care)
Flatworms are thin, soft-bodied animals (often only a few millimeters long) that glide over surfaces. In reef aquariums, they matter for two reasons:
- They can multiply fast when conditions favor them.
- Some species irritate or eat corals, and large die-offs can foul the water.
Flatworms vs. other pests (nudibranchs, worms, pods)
Beginner ID tips:
- Flatworms: very thin, “leaf-like,” glide smoothly, often oval/teardrop shaped.
- Nudibranchs: usually thicker, have visible appendages/cerata, move more like a slug.
- Bristle/other worms: segmented bodies, often hide in rock/sand, not flat “discs.”
- Pods (copepods/amphipods): tiny “bug-like” movement, darting or hopping.
If it glides like a tiny sticker across glass or coral, flatworms become more likely.
Are all flatworms harmful?
No. Some are mostly a nuisance in low numbers. Others (notably Montipora-eating flatworms, MEFW) are a real coral threat. Even nuisance flatworms can become risky if they explode in population and then die all at once.
Common flatworms you may see in marine aquariums
Red planaria (acoel flatworms) on glass and rocks
These are the classic “red planaria flatworms” (often called acoel flatworms). You may see:
- Reddish/rust colored specks on glass, rocks, and sand
- Clustering in low-flow areas
- Increased numbers when nutrients and film algae are available
They often look worse than they are at first—but they can shade corals or irritate tissue when they pile up.
Small “rust/brown” flatworms on sand and surfaces
You might see tan/rust/brown flatworms that blend into sand and rock. Behavior is similar: they glide, cluster, and multiply when they have plenty of food (films, detritus, and microalgae).
Montipora-eating flatworms (MEFW) on corals
MEFW are a different category. They target Montipora (caps, plates, encrusting). They’re harder to spot than red planaria because they blend in and spend time on coral surfaces and undersides.
If you keep Montipora, treat any suspicious bite marks seriously.
When it’s not flatworms: detritus mats and diatoms
Before you panic, rule out:
- Diatoms: dusty brown coating, wipes away in sheets/film, common in newer tanks
- Cyanobacteria mats: slimy sheets with bubbles
- Detritus mats: “dirty” film that blows off in clouds
Flatworms usually have a distinct “gliding animal” look rather than a uniform coating.
How to identify flatworms (beginner-friendly checks)
Visual signs (shape, movement, clustering)
Look for:
- Teardrop/oval shape
- Smooth gliding motion (not hopping)
- Clustering in corners, low flow zones, or shaded areas
- Color: red/rust/brown; sometimes translucent
Where to look: glass, overflow, underside of frags
Best places to check:
- Front glass (especially lower corners)
- Overflow box and weir teeth
- Back side/underside of coral frags and plug edges
- Shaded rock overhangs
Confirming MEFW: bite marks, egg clusters, flashlight test
MEFW signs on Montipora:
- “Bite marks”: pale patches or irregular missing tissue areas
- Reduced polyp extension on affected areas
- Egg clusters: tiny, clear/white patches usually on the underside or protected edges
- Flashlight test: check after lights out (or with blues down). Use a small flashlight and look along edges and underside.
Safety note: avoid blasting corals repeatedly with strong jets—gentle inspection is better than constant stress.
Photo documentation and tracking (what to record in Reef Buddy)
A simple tracking habit helps you avoid guessing. In Reef Buddy, log:
- First date you noticed them
- Where they appear (glass, sand, Montipora edges)
- Photos weekly from the same angle
- Actions taken (siphoning, filter sock, carbon, treatment date)
- Key parameters (see checklist below)
This makes it easier to see whether the population is growing, stable, or shrinking.
Why outbreaks happen
Flatworms thrive when food is abundant and predators/competition are low.
Nutrients and feeding: excess detritus and film algae
Common triggers:
- Heavy feeding without matching export
- Dirty filter socks/media traps
- Detritus buildup behind rocks or in the sump
- Strong film algae growth (a food source)
Reference ranges many beginner reefs aim for (varies by tank goals):
- Nitrate (NO₃): ~2–20 mg/L
- Phosphate (PO₄): ~0.03–0.15 mg/L
Chasing “zero” nutrients can destabilize corals. Focus on consistency.
Low flow and “dead spots”
Flatworms often cluster where flow is weak. Dead spots also trap detritus, feeding the problem. Adjust pumps so debris stays suspended long enough to be filtered.
Lack of quarantine and hitchhikers
Most pest flatworms arrive on:
- Coral bases and plugs
- Frag rack surfaces
- Macroalgae or rubble
- Shared tools between tanks
Stability issues (temperature, salinity swings) that stress corals
Stressed corals are easier targets. Common stability targets:
- Temperature: ~25–26°C (77–79°F), avoid swings >0.5°C (1°F) daily
- Salinity: ~35 ppt (1.026 SG), avoid sudden changes
- Alkalinity: often ~7–9 dKH for mixed reefs (pick a target and keep it stable)
Are flatworms dangerous?
When they are mostly a nuisance (low numbers)
If you see a few on glass and rocks and corals look fine, you can often start with:
- Siphoning during water changes
- Improving flow and cleanliness
- Monitoring growth
When they become a real problem (corals closing, shading, tissue loss)
Act more aggressively if you notice:
- Corals staying closed
- Flatworms carpeting coral tissue (shading/irritation)
- Montipora showing bite marks
- Rapid population growth week-to-week
The toxin risk: die-off events and why carbon matters
Some flatworms (especially planaria-type outbreaks) can release irritating compounds when they die in large numbers. The risk is not the treatment itself—it’s the mass die-off.
Safety basics before any major kill-off:
- Run fresh activated carbon
- Have mixed saltwater ready for a large water change
- Increase aeration/oxygenation (aim powerheads at the surface)
- Be ready to remove dead worms fast (siphon + filter sock)
Step-by-step control plan (safe for beginners)
Step 1 — Reduce numbers mechanically
This lowers toxin risk and often solves mild cases without meds.
- Siphon during water changes:
Use airline tubing for precision. Target clusters on glass and sand. - Turkey baster “blast” on rocks/corals:
Gently dislodge them so they can be filtered/siphoned. - Filter sock strategy:
Add a fine sock temporarily (e.g., 100–200 micron) while you blow/siphon. Replace/clean it the same day so it doesn’t become a nutrient trap.
Tip: do this in short sessions to avoid stressing fish and corals.
Step 2 — Improve conditions that favor control
Simple changes that reduce food and habitat:
- Increase random flow to eliminate dead spots
- Blow detritus off rocks weekly (then remove it via filtration/skimmer)
- Review feeding: reduce “just in case” feeding, rinse frozen foods
- Maintain export: skimming, refugium, or regular water changes
Step 3 — Biological options (predators): pros/cons
Some hobbyists use predators as part of reef tank pest control, but they’re not guaranteed and may create compatibility issues.
Commonly discussed options (availability and behavior vary):
- Certain wrasses (some may pick at flatworms)
- Some dragonets (mostly eat pods; may not help much)
- Other specialty feeders (often not beginner-friendly)
Compatibility warnings:
- Don’t add a fish you can’t keep long-term.
- Many “predators” won’t eat flatworms once they learn easier foods.
- Some may harass peaceful tankmates or require a mature pod population.
Why predators are not a “cure”: they rarely eliminate eggs and may only reduce visible adults.
Step 4 — Targeted treatments (when needed)
If the population is growing fast, corals are affected, or mechanical control isn’t keeping up, consider a targeted flatworm treatment. Follow the product instructions exactly.
Planning checklist (beginner-safe):
- Mix enough saltwater for a large water change (often 25–50% depending on severity)
- Add fresh activated carbon and have extra ready
- Increase aeration (surface agitation, air stone in sump if safe)
- Prepare siphon tubing and clean filter socks
- Keep test kits ready (especially ammonia)
About “flatworm exit dosage”:
Many products have dosing guidance by tank volume. Dose based on actual water volume (tank minus rock/sand displacement) when possible. If you’re unsure, calculate conservatively and confirm with the label.
Re-treatment logic:
- Treatments may not affect eggs, so you may need a second round per instructions.
- Always remove as many worms as possible before dosing to reduce die-off pollution.
What to do if fish/corals react:
- Stop and stabilize: add carbon, increase aeration, do a water change.
- Keep hands out of the tank if you suspect oxygen is low—fix gas exchange first.
Safety note: never mix treatments. Avoid “stacking” chemicals (including multiple pest products) unless a trusted manufacturer protocol explicitly says it’s safe.
Step 5 — Post-treatment cleanup
This step prevents the “treatment worked… then the tank crashed” scenario.
- Run fresh activated carbon (replace after it loads up)
- Do a large water change per product guidance (often within hours)
- Empty/clean mechanical filtration frequently for 24–48 hours
- Monitor:
- Ammonia: should remain 0 mg/L
- Fish breathing rate (oxygen stress sign)
- Coral response over the next 2–7 days
MEFW-specific response (Montipora keepers)
If you suspect montipora eating flatworm (MEFW), treat it like a coral quarantine problem, not a “dose the display and hope” problem.
Immediate actions (isolate, inspect, frag if necessary)
- Move affected Montipora to a quarantine tank if possible
- Inspect undersides, edges, and shaded areas
- Consider fragging away healthy pieces if tissue loss is progressing
Coral dips and why eggs are the challenge
Dips can remove or kill some mobile flatworms, but eggs are the hard part. Eggs may survive many dips, which is why repeat inspections are critical.
Quarantine schedule and repeat dips
A common beginner-friendly approach is:
- Dip + inspect
- Remove any visible eggs manually (gentle scraping on plug/rock, not living tissue)
- Repeat dip/inspection on a schedule (often every few days to weekly, depending on the dip and guidance)
- Continue quarantine long enough to outlast hatch cycles
Follow your dip product instructions for timing and concentration. Rinse corals with clean saltwater before returning to quarantine.
When to consider removing Montipora temporarily
If MEFW is confirmed in the display and you can’t fully treat in place, removing Montipora to quarantine can help break the cycle. This is situational, but it’s sometimes the safest path for the rest of the reef.
Prevention: keep flatworms out long-term
Quarantine workflow for new corals (beginner version)
A simple, repeatable workflow:
- Place new coral in a separate quarantine tank (even a basic setup)
- Inspect with white light and a magnifier
- Dip per coral dip instructions
- Observe for several weeks, inspecting undersides regularly
- Only then move to display
This is the single best way to “quarantine corals to prevent pests.”
Dip protocol overview (what dips can and can’t do)
Dips can help with:
- Mobile pests on coral surfaces
- Some hitchhikers on plugs and frag racks
Dips usually cannot be trusted to:
- Reliably kill all eggs
- Fix pests already established in the display
Use dips as a layer of defense, not the only one.
Routine inspections and “pest calendar”
Add a weekly 5-minute inspection:
- Glass corners
- Overflow
- Undersides of frags (especially Montipora)
- Rock crevices in low flow areas
A simple “pest calendar” reminder prevents problems from becoming surprises.
Parameter stability checklist (and how tracking helps)
Stability supports coral resilience and reduces outbreak fuel.
Beginner stability targets to track:
- Salinity: 35 ppt
- Temperature: 25–26°C (77–79°F)
- Alkalinity: 7–9 dKH (choose a target and keep it steady)
- Nitrate: ~2–20 mg/L
- Phosphate: ~0.03–0.15 mg/L
Tracking trends (not just single numbers) helps you spot the “why now?” behind outbreaks.
Flatworm control options (quick comparison)
| Approach | Best for | Pros | Cons / Risks | Beginner-friendly? |
|---|---|---|---|---|
| Siphoning + baster | Low to moderate planaria | Low risk, immediate reduction | Takes time, won’t stop eggs | Yes |
| Flow + detritus cleanup | Preventing recurrence | Improves tank overall | Slower results | Yes |
| Predators (fish/inverts) | Long-term pressure | Can reduce visible worms | Unreliable, compatibility issues | Sometimes |
| Targeted chemical treatment | High numbers, fast spread | Can knock down outbreak quickly | Die-off toxin risk, requires planning | With preparation |
| Quarantine + dips (MEFW) | Coral-specific pests | Protects display long-term | Requires extra tank and routine | Yes (with simple setup) |
FAQ: flatworms in reef aquariums
What causes flatworms in a reef tank?
Usually a mix of hitchhikers (no quarantine) plus favorable conditions like detritus, film algae, and low-flow areas. Once established, they can multiply quickly.
Are red planaria flatworms harmful?
In small numbers, they’re often a nuisance. In large numbers, they can shade and irritate corals, and a mass die-off can degrade water quality. Treat outbreaks carefully.
How do I get rid of flatworms without harming corals?
Start with the safest steps: siphon, improve flow, remove detritus, and reduce excess feeding. If you must treat, plan for carbon, aeration, and water changes to manage die-off.
Do flatworm treatments kill fish or invertebrates?
Some treatments can stress livestock, but the bigger danger is often oxygen drop and pollution during die-off. Prepare with strong aeration, remove worms first, run carbon, and follow the label exactly.
What eats flatworms in a saltwater aquarium?
Some fish may pick at them, but results vary. Predators are best viewed as support, not a guaranteed cure—especially because they don’t reliably address eggs.
Will coral dips remove flatworm eggs?
Usually not reliably. Dips can help remove mobile pests, but eggs often survive, which is why repeat inspections and quarantine are so important.
When to ask for help (and what info to share)
Reach out for experienced help if:
- Montipora tissue loss is progressing
- Fish are breathing fast after any treatment attempt
- You can’t keep ammonia at 0 mg/L
- Flatworms return quickly after multiple control attempts
Share:
- Clear photos (white light + close-up)
- Tank size and estimated water volume
- Full livestock list
- Recent changes (new coral, feeding change, parameter swings)
- Key parameters:
- Salinity (ppt)
- Temperature (°C/°F)
- Nitrate (mg/L)
- Phosphate (mg/L)
- Alkalinity (dKH)
CTA: Make flatworm control easier to manage
Flatworms are much easier to beat when you can see patterns—when they appeared, what changed, and whether your plan is working. Track your parameters, maintenance, and photo notes in Reef Buddy, and use Shrimpy as your friendly checklist brain for your next siphon session, dip schedule, or treatment prep.