Nudibranchs can be one of the most confusing “pests” in reef tanks—because some are harmless sightings, while others can wipe out a favorite coral quickly. This beginner guide will help you identify nudibranchs (and their eggs), recognize coral damage patterns, and take safe, practical removal steps without guessing.
What Are Nudibranchs?
Quick basics (sea slugs, not “worms”)
Nudibranchs are marine gastropods (sea slugs). They are not worms and not the same as flatworms. Many have:
- A soft, slug-like body
- Rhinophores (two antenna-like “feelers” on the head)
- Cerata (frilly spikes or tufts on the back in many species)
Why they show up in aquariums (hitchhikers on corals/live rock)
In reef aquariums, nudibranchs usually arrive as hitchhikers on:
- Coral frags (especially on the underside of plugs)
- Encrusted coral bases
- Live rock attached to frags
- Macroalgae or rubble added from other systems
Are Nudibranchs Good or Bad in a Reef Tank?
The key rule: most are specialized feeders
Most nudibranchs are specialized—they eat a specific food source. That’s why the “same-looking” nudibranch in two tanks can be harmless in one tank and destructive in another.
When they’re harmless (detritus/algae-associated sightings)
Sometimes a nudibranch is simply passing through, especially if:
- You see one and never see it again
- There’s no matching coral damage
- It’s on the glass/rock and not staying on a specific coral colony
Still, treat any nudibranch as “unknown” until you’re confident it isn’t feeding on your corals.
When they’re a problem (coral predators, rapid coral decline)
Red flags that suggest a harmful nudibranch issue:
- Coral tissue loss that starts at edges or shaded areas
- Decline that continues despite stable parameters
- Multiple sightings, especially at night
- Visible egg ribbons near the affected coral
Common Nudibranch Types Beginners Encounter
Montipora-eating nudibranchs (MEN)
Montipora-eating nudibranchs are among the most common coral-predator nudibranchs in reef tanks.
What they look like (typical traits):
- Small (often just a few mm when young)
- White/cream bodies that can blend with Montipora
- Frilly backs (cerata) that make them look “fuzzy”
Typical damage patterns on Montipora:
- Patchy tissue loss starting on edges/undersides
- “Clean” areas where tissue is gone and skeleton shows
- Often worst on plating/encrusting Montipora where they can hide underneath
Zoanthid-eating nudibranchs
These nudibranchs can be very hard to spot because they often:
- Match the color of the zoas/palys they’re eating
- Hide between polyps and along the mat
- Come out more at night
Typical signs on zoas/palys:
- Polyps staying closed for days
- “Melting” or patchy disappearance
- Damage that spreads from one section/frag to another
Safety note: When handling zoanthids/palythoa, use gloves and eye protection and avoid aerosolizing water (no hot water, no scrubbing that creates spray). This is general handling safety, not medical advice.
“Pretty” nudibranchs you should still treat cautiously
Bright colors don’t mean “reef safe.” Some beautiful nudibranchs still:
- Feed on corals or sponges
- Carry toxins from their food sources
- Die in pumps or overflows and foul water
If you can’t confirm diet and identity, treat it as a potential risk.
How to Identify Nudibranchs (Without Guessing)
Visual markers (shape, cerata, rhinophores)
Look for these nudibranch clues:
- Two head “feelers” (rhinophores) that look like short antennae
- A slug body with a distinct head end
- Frilly spikes/tufts (cerata) on the back (common, not universal)
Tip: Flatworms are usually flatter and glide like a sheet; nudibranchs look more 3D and “sluggy.”
Where to look (undersides, edges, shaded areas)
Check the areas nudibranchs prefer:
- Undersides of frag plugs and coral bases
- Shaded crevices and overhangs
- Coral edges where tissue meets skeleton
- Between zoa polyps and along the mat
Night checks (why flashlight inspections work)
Many coral-predator nudibranchs are more active after lights out. Do a quick inspection:
- 1–2 hours after your display lights go off
- With a small flashlight (red mode if you have it)
- Focusing on the coral you suspect first
Eggs: what they look like and why they matter
Nudibranch eggs often appear as:
- Spiral coils
- Ribbon-like or “stringy” gelatinous patches
- Placed on the underside/edges near the food coral
Why eggs matter:
- Many dips do not reliably kill eggs
- If you remove adults but miss eggs, the problem returns fast
Remove egg masses immediately.
Signs Your Corals Are Being Targeted
Montipora: tissue loss patterns and “bite marks”
Common Montipora signs include:
- Tissue loss that starts on the underside and creeps outward
- Small, irregular patches that expand over days
- Reduced polyp extension in affected zones
Zoas: closed polyps, melting, patchy decline
Look for:
- A cluster of polyps that stay closed while neighbors look okay
- Gradual “melting” in a line or patch
- New frags declining first (often where pests arrived)
Ruling out lookalikes (flatworms, amphipods, starvation, light shock)
Before you blame nudibranchs, check:
- Parameter swings (especially alkalinity)
- Light shock (new placement under stronger PAR)
- Flow issues (detritus settling, cyano irritating tissue)
- Lookalikes: acoel flatworms, certain small amphipods, and general recession from stress
Helpful stability ranges (typical reef targets):
- Temperature: 25–26°C (77–79°F)
- Salinity: 35 ppt (1.026 SG)
- Alkalinity: 7–9 dKH (keep stable)
- Nitrate: 2–20 mg/L
- Phosphate: 0.03–0.10 mg/L
Stability won’t “cure” pests, but it helps corals survive the cleanup process.
Safe Removal and Control (Beginner-Friendly)
Manual removal (tweezers, siphon, turkey baster)
Start simple and controlled:
- Turkey baster: blast gently to dislodge nudibranchs from coral surfaces
- Siphon tube: remove dislodged pests immediately (don’t let them float elsewhere)
- Tweezers: grab visible adults carefully
Safety notes:
- Avoid tearing coral tissue while removing pests.
- Dispose of removed pests/eggs outside the tank water (don’t release in sump).
Coral dips: what they help with—and what they don’t
Dips can help knock off or kill mobile pests, but they may miss:
- Eggs
- Pests deep in crevices or under encrusted bases
Common beginner approach:
- Use a reputable coral dip product per label directions.
- Dip in a separate container with tank water.
- Rinse coral in a second container of clean tank water before returning to QT.
Important: Never mix dip chemicals, and don’t exceed label time/concentration.
Repeat schedule matters
Because eggs can hatch later, repeat dips are often needed:
- Dip, inspect, remove eggs
- Repeat every 4–7 days for 2–3 weeks (depending on what you’re seeing)
This helps catch newly hatched nudibranchs before they lay more eggs.
Managing eggs (scrape, cut away, re-dip)
Egg removal is often the turning point. Options:
- Scrape egg ribbons off plugs/bases with a pick or blade (outside the tank)
- Cut away the frag plug rim or remove the coral from the plug if practical
- Re-dip after physical removal
If eggs are on a heavily encrusted base you can’t clean well, consider isolating or sacrificing the base to protect the rest of the tank.
Quarantine workflow for new corals
A simple coral quarantine (QT) setup prevents most nudibranch disasters:
- Small tank or container with heater, small powerhead, simple light
- Frag rack for easy viewing
- Dedicated tools (tweezers, baster, magnifier)
Easy routine:
- Day 1: Dip + detailed inspection (underside, plug, crevices)
- Days 4–7: Re-dip + inspect for eggs
- Repeat weekly for 2–4 weeks before moving to display
Prevention: Best Practices That Actually Work
Buying and trading corals safely
Reduce risk by:
- Asking if the seller quarantines and dips routinely
- Avoiding freshly imported or freshly cut frags when possible
- Inspecting frags under white light before purchase
Inspection checklist before placement
Use this quick checklist:
- Underside of plug/base checked
- Edges of tissue inspected for egg ribbons
- Coral blasted with baster in a clear container
- Any suspicious patch scraped/removed
- Coral dipped and rinsed
Tracking coral health and parameters
Corals recover best when your fundamentals are stable. Track:
- Alkalinity (dKH) stability (avoid big corrections)
- Temperature stability (avoid daily swings)
- Nutrients (avoid bottoming out to 0)
Reef Buddy can help you log dKH, ppt, mg/L values and spot trends early—useful when you’re trying to tell “pest damage” from “parameter stress.”
When to Escalate (and What to Avoid)
If the infestation persists after dips
If you’re still seeing nudibranchs after repeated dips and egg removal:
- Move the affected coral to QT (if it’s not already)
- Increase inspection frequency (especially night checks)
- Consider removing the coral from the plug/base to eliminate hidden eggs
Avoid “reef safe” miracle cures
Be cautious with products that promise to “wipe out pests” in a reef-safe way. Many can:
- Stress corals
- Harm invertebrates
- Cause oxygen depletion if they kill lots of organisms at once
Stick to controlled methods: QT, manual removal, repeated dips, egg scraping.
When to consider removing affected colonies to save the rest
If a colony is heavily infested and you can’t control eggs:
- Removing the colony may protect the rest of the system
- Fragging healthy sections into clean plugs (then quarantining) can sometimes save part of it
Comparison: Control Options at a Glance
| Method | Best for | Pros | Cons / Limits | Beginner-friendly? |
|---|---|---|---|---|
| Manual removal (baster/siphon/tweezers) | Visible adults | Immediate, cheap | Misses eggs; time-consuming | Yes |
| Coral dip (per label) | Mobile pests on frags | Helps reduce numbers fast | Often misses eggs; requires repeats | Yes |
| Egg scraping/cutting plug | Egg masses | Stops the life cycle | Can damage coral if rushed | Yes (with care) |
| Quarantine (2–4 weeks) | Prevention + treatment | Most reliable long-term | Requires space/equipment | Yes |
| “Reef safe” tank-wide cures | Broad pests (claims) | Easy to dose | Unpredictable, can stress livestock | Not recommended |
Quick Action Plan (Printable Summary)
24-hour response
- Inspect the affected coral closely (underside + edges).
- Do a night flashlight check.
- Manually remove any nudibranchs you see (siphon them out).
- Dip the coral (follow product directions).
- Scrape/remove any egg ribbons immediately.
- Move the coral to QT if possible.
2-week follow-up
- Re-dip every 4–7 days.
- Inspect for new eggs each time (remove on sight).
- Track coral response and stability:
- Temp 25–26°C (77–79°F)
- Salinity 35 ppt
- Alk 7–9 dKH (stable)
Long-term prevention routine
- Quarantine all new frags for 2–4 weeks.
- Dip + inspect on a schedule (don’t rely on a single dip).
- Keep basic parameters stable and logged.
- Treat every “mystery slug” as a potential specialist feeder until proven otherwise.
FAQ
What is a nudibranch and why is it in my reef tank?
A nudibranch is a sea slug. In reef tanks, they usually arrive as hitchhikers on coral frags, frag plugs, or attached rock.
Are nudibranchs harmful to corals?
Some are harmless, but many are specialized feeders. If one is a coral predator (like a Montipora- or zoa-eating type), it can cause fast tissue loss and repeated decline.
How do I identify a Montipora-eating nudibranch?
Look for tiny white/cream, frilly-backed nudibranchs on Montipora—especially on the underside or along edges—plus patchy tissue loss that spreads over time.
What do nudibranch eggs look like in an aquarium?
They’re often spiral or ribbon-shaped gelatinous masses attached near the food coral, commonly on undersides and shaded edges. Remove them right away.
Do coral dips kill nudibranchs and their eggs?
Dips often help with mobile nudibranchs, but eggs can survive. That’s why manual egg removal and repeat dips are important.
How often should I dip corals to stop nudibranchs?
A common beginner schedule is every 4–7 days for 2–3 weeks, combined with careful egg scraping each time. Adjust based on what you observe in QT.
CTA: Make Pest Control Less Stressful
If you’re dealing with nudibranchs, your best advantage is consistency: repeat inspections, stable parameters, and a clear schedule. Reef Buddy makes it easier to log alkalinity (dKH), salinity (ppt), nitrate/phosphate (mg/L), and coral notes so you can see what’s changing while you treat pests. If you want a simple plan, open Reef Buddy and let Shrimpy help you set a dip/QT reminder routine and track recovery day by day.