Most reef tank pests arrive as hitchhikers on new corals or rock, and almost all of them can be managed with patience. The usual options are manual removal, a natural predator, coral dips, and, above all, quarantining new additions so the next pest never gets in. Below is a short guide to identifying each common pest, how much it matters, and the options keepers use.
First, take a calm look
Finding something new in the tank does not mean you have a problem. Many of the creatures people spot at night with a flashlight are part of a healthy reef. Before treating anything, take a clear photo, note where it lives (rock, sand, coral tissue, coral base), and watch whether nearby corals are closing up or losing tissue. A pest that is causing no harm can often be watched for a week or two while you decide.
Aiptasia (glass anemones)
Identify: Small, translucent brown anemones, usually 0.25 to 1 inch across, with long thin tentacles. They retract into holes in the rock when touched.
Impact: They sting nearby corals and can spread quickly when food is plentiful. A few are a nuisance; dozens compete for space.
Options:
- Manual: Inject or cover each anemone with a kalkwasser paste or a dedicated aiptasia treatment. Turn off flow first so the treatment stays on target. Avoid scraping or cutting them off, since fragments can regrow into new anemones.
- Predators: Peppermint shrimp are the classic choice, though some individuals ignore Aiptasia, and they may pick at other polyps once the anemones are gone. Berghia nudibranchs eat only Aiptasia, so they work well but starve once the job is done. An Aiptasia-eating filefish can clear a tank fast, but it may also nip zoanthids, soft corals, and clam mantles.
- Patience: Treat in small rounds over several weeks rather than all at once.
Majano anemones
Identify: Similar in size to Aiptasia, but with short, stubby, often green or pink tentacles and a bulbous look. They tend to sit in the open rather than hide in holes.
Impact: Like Aiptasia, they sting and spread, and they can be harder to shift.
Options: Manual treatment with kalkwasser paste or an anemone treatment is the most reliable route. Berghia nudibranchs and peppermint shrimp generally leave Majanos alone. Some filefish will eat them, with the same trade-offs to soft corals and zoanthids noted above.
Red planaria (flatworms)
Identify: Tiny rust or red-brown flatworms, about 0.1 to 0.25 inch long, often with a small fork-shaped tail. They gather on sand, rock, and glass, usually in the brightest spots.
Impact: In small numbers they are mostly cosmetic. In large numbers they can carpet corals and block light.
Options:
- Manual: Siphon them out during water changes. This alone keeps light populations in check.
- Treatments: Flatworm treatments are available. If you use one, siphon out the dead worms quickly, run fresh carbon, and have saltwater ready for a water change, because dying flatworms in large numbers can release toxins into the water.
- Predators: Some small wrasses and certain flatworm-eating nudibranchs will eat them. The nudibranchs have a narrow diet and do not last long once the worms are gone.
- Food supply: Populations often swell when there is plenty of food and light. Feeding a little less can help. Avoid stripping nutrients too far to starve them, though; see our guide on how to lower nitrate and phosphate safely.
Acropora-eating flatworms (AEFW)
Identify: Small, flat, translucent tan worms that match Acropora tissue almost perfectly. You usually notice the damage first: rounded bite marks and patches of missing tissue. Their egg clusters look like small brown or golden dots on bare skeleton near the coral base.
Impact: AEFW are the pest Acropora keepers watch most closely, since a heavy infestation can strip colonies over time.
Options:
- Dips: Dip affected corals in a coral dip and blow off the worms with a turkey baster. Dips do not kill the eggs, so repeat the dip over several weeks to catch each new hatch.
- Remove eggs: Inspect the base and underside of each colony and scrape off any egg clusters, or snip off the bare skeleton they sit on.
- Predators: Some wrasses pick at them, but no fish reliably clears an infestation on its own.
Red bugs
Identify: Very small yellow bodies with a red-orange dot, roughly 0.04 inch across, sitting on Acropora branches. A magnifying glass helps.
Impact: Affected corals often show poor polyp extension and slowed color or growth.
Options: Dips reduce numbers. A full cure usually means treating the tank with a product that targets crustaceans, which also kills shrimp, crabs, copepods, and amphipods. For that reason, many keepers move Acropora to a separate tank for treatment rather than dose the display. Talk to an experienced reefer or your local store before using any treatment of this kind.
Montipora-eating nudibranchs
Identify: Small white or pale nudibranchs that sit on the underside of plating and encrusting Montipora. Look for white spiral egg ribbons nearby.
Impact: Tissue loss that starts at the edges or underside of the plate and slowly spreads.
Options: Dip the coral, remove every adult you can see, and scrape off the egg ribbons. As with AEFW, repeat dips over several weeks, since eggs hatch after the first round.
Zoanthid-eating nudibranchs and sea spiders
Identify: Zoa-eating nudibranchs are small and closely match the color of the polyps they feed on, so they are easy to miss. Sea spiders are thin, long-legged creatures, often under 0.25 inch, that cling between polyps. Both leave clusters of polyps closed or shrinking.
Impact: A colony that stays closed for days, while other zoanthids in the tank are open, is the usual sign.
Options:
- Dips: A coral dip, followed by a strong blast with a baster, removes most adults.
- Manual: Pick off what you can see with tweezers, and look for egg masses on the rock or plug.
- Repeat: Dip again over the following weeks to catch any that hatch.
Vermetid snails
Identify: Snails that live in hard, stationary tubes cemented to rock or glass. They cast sticky mucus threads into the water to catch food.
Impact: The mucus nets can irritate corals growing near them, which then stay closed or lose tissue at the contact point.
Options: There is no reliable predator. The usual method is to crush the tube opening with a tool and then seal it with reef-safe glue or epoxy, or cover it with kalkwasser paste. Populations drop when there is less suspended food, so steady, moderate feeding helps over time.
Usually harmless: watch, do not treat
Bristle worms
Most bristle worms are useful scavengers that eat leftover food and detritus. A large population usually means there is extra food in the tank, not that the worms are a problem. The exception is large fireworms, which can grow several inches long and may eat corals or small invertebrates. Remove those with a trap. Wear gloves when handling rock, since the bristles can sting.
Copepods and amphipods
These small crustaceans are food for fish and corals, and many keepers add them on purpose. Seeing lots of them on the glass at night is a good sign.
Asterina stars
Asterina are tiny starfish, usually under 0.5 inch. Most are harmless grazers on algae and film. Some keepers report certain types eating zoanthid or Acropora tissue, and this is still debated in the hobby. If you see stars on a coral that is losing tissue, remove them by hand. Harlequin shrimp eat starfish, but they eat nothing else, so they will need a steady supply of stars once the Asterina are gone.
Prevention: quarantine and dip new corals
Almost every pest above arrives on a new coral, frag plug, or piece of rock. A simple routine keeps most of them out:
- Inspect every new coral under bright white light, including the plug and the underside.
- Dip it in a coral dip, following the product directions, and blow off anything that comes loose.
- Remove the frag plug or bare skeleton where you can, since eggs often sit there.
- Quarantine new corals in a separate tank for several weeks, repeating dips during that time. Many keepers hold corals for 4 to 8 weeks to cover the egg cycles of AEFW and nudibranchs.
If you are starting a new tank, dry rock avoids most hitchhikers from day one. Our guide on how to cycle a new reef tank covers that choice.
Track pests with Reefability
If you are not sure what you are looking at, the Reefability Issue Identifier can suggest a likely match from a photo. Once you know what it is, you can log the pest against your tank with a treatment plan and follow-up reminders, so you can see whether the numbers are falling over the following weeks. You can also ask Reefer AI how a new pest or treatment fits with the livestock you already keep.
Frequently asked questions
Do peppermint shrimp really eat Aiptasia?
Many do, but not all. Results vary by individual, and once the Aiptasia are gone some shrimp may pick at other polyps. Berghia nudibranchs are more reliable but starve afterward.
Are bristle worms good or bad for a reef tank?
Mostly good. They clean up leftover food and detritus. Only large fireworms are worth removing, and a population boom usually points to extra food in the tank.
How often should I dip corals for AEFW?
Dips kill adults but not eggs, so most keepers repeat them over several weeks while also scraping off egg clusters. Inspect the coral closely each time and continue until you find no worms or eggs.