Coral Pests 101: How to Identify and Treat the Bugs Eating Your Coral

Most coral losses in a reef tank aren’t caused by water chemistry — they’re caused by something eating the coral. Coral pests are small, often nearly invisible, and by the time you notice the damage they’ve usually been at work for weeks. The good news: every one of them has a tell, and almost all of them are beaten the same way — quarantine and dip every new coral before it touches your display. This guide walks through the pests that actually eat coral tissue, how to identify each one, and how to treat them. (For the rock-and-glass nuisance pests — Aiptasia, Asterina stars, hydroids, and vermetid snails — see our companion guide on common reef tank pests.)

A note on the word “insects.” Plenty of people arrive here searching for coral insects or bugs in a reef tank, which is exactly the right instinct and the wrong word. There are essentially no insects in the ocean. What is eating your coral is a flatworm, a nudibranch, a snail or a tiny crustacean — red bugs, for example, are copepods, not bugs. It matters practically, not just pedantically: an insecticide-style “treat the whole tank” reflex is what kills reef tanks, while each of the animals below has its own dip, predator or manual removal that works. Find the animal first, then treat it.

Zoanthids specifically? Pests, zoa pox and palytoxin handling are covered in depth in why are my zoanthids not opening.

First: how to tell it’s a pest and not something else

Coral tissue recedes for many reasons — light shock, alkalinity swings, low nutrients, aggression from a neighbor. Before you assume pests, look for the specific signatures below. The universal red flags of a predator are: tissue loss that starts at the base or underside and creeps upward, bare white skeleton with no algae yet (fresh eating, not old die-off), bite marks or missing color in patches, and coral that looks fine in the day but worse each morning (many pests feed at night). When in doubt, pull the coral and dip it — you’ll usually see the culprits fall off into the dish.

The SPS predators (Acropora, Montipora & friends)

Acropora-Eating Flatworms (AEFW)

What to look for: AEFW on Acropora — flat oval worm, ~3–6 mm, nearly transparent, takes on the coral’s color

What they are: Amakusaplana acroporae, a flat oval polyclad flatworm that targets Acropora up to about 6 mm long. They are the single most feared Acropora pest because they are almost perfectly camouflaged — they take on the exact color of the coral they’re eating.

How to ID them: You rarely see the worm itself. You spot the damage and the eggs. Look for “bite marks” — small patches at the base of the branches where the color/zooxanthellae are missing, leaving pale spots. The give-away is the eggs: tiny rust-colored to brown dots laid in clusters on the bare skeleton at the base of the colony. Encrusted, browning-out acros that won’t color up despite good water are a classic sign. A freshwater or coral dip will make the adults curl off — they look like flat, clear-to-tan teardrops in the dish.

Why they’re so hard to kill: Dips kill adults but do not kill the eggs, which are protected by a shell and hatch days later. That’s why one dip never works.

Treatment: Remove every Acropora (including encrusted bits on the rock) and run a repeating dip cycle — coral dip or a freshwater dip — every 4–7 days for at least 6 weeks, so each new hatch is killed before it can lay. Between dips, blow the eggs off the bases with a turkey baster. Some keepers add a Bayer or Coral Rx dip on intake. Wrasses (six-line, melanurus, leopard) help control adults but won’t clear an infestation alone.

Red Bugs (Tegastes acroporanus)

What to look for: Red bugs — tiny yellow flea-like copepods with a red dot, scattered on Acropora tissue

How to ID them: Look closely (a magnifier or phone macro helps) at the tissue of your Acropora, especially smooth-skinned species. Red bugs are tiny yellow, flea-shaped copepods, each with a single red dot, that scuttle across the coral. Affected acros stop extending polyps, lose color, and “brown out” even though water parameters are fine. They don’t leave bite marks like flatworms; the damage is stunting and unhappiness.

Treatment: The classic cure is a milbemycin-based treatment (e.g. Interceptor) dosed to the whole system, usually three treatments spaced a few days apart — but note it kills all crustaceans (shrimp, crabs, pods), so remove them first. A Bayer dip on intake also knocks them down and is a good preventive.

Black Bugs / Gray Bugs

How to ID them: A less common but nastier acro parasite. Unlike red bugs (which brown the coral out), black/gray bugs make the coral look bleached and eaten, with pale, chewed-looking patches. The bugs themselves are dark specks on the tissue. They can affect Acropora, Pocillopora, Merulina, and other SPS. Because they’re rarer, they’re often missed until the coral is badly damaged.

Treatment: More stubborn than red bugs. A strong dip regimen (Bayer) on quarantine, and in-system treatments are still debated — quarantine and repeated dipping are your safest bet. When in doubt, isolate the affected coral.

Montipora-Eating Nudibranchs

What to look for: Montipora-eating nudibranch — tiny white sea slug, ~few mm, matching the coral, with white spiral egg coils

How to ID them: They feed only on Montipora and Anacropora, and they are tiny (a few mm), white, and shaped to blend into the coral. You’re more likely to see their work than the slug: advancing bands of bare white skeleton, often starting on the underside or shaded areas, plus tiny white spiral egg ribbons tucked on the skeleton. They breed explosively — about 100 eggs at a time, hatching in 36–96 hours, adult in under a week — so a small problem becomes a dead colony fast.

Treatment: Dip every Montipora on intake and repeat every 4–8 days until no more slugs appear (dips don’t kill eggs). Blow off egg coils between dips. Wrasses help. For a bad infestation, remove all Montipora and run the dip cycle for 6+ weeks.

The zoanthid & palythoa predators

Zoanthid-Eating Nudibranchs (“Zoa Nudis”)

What to look for: Zoa-eating nudibranch — camouflaged slug that mimics the zoanthid’s color, hiding among the polyps

How to ID them: The ultimate copycat. As these nudibranchs eat your zoas, they absorb the polyps’ pigment and even fluorescence, until they look like another head in the colony. Signs: zoa polyps closing up and melting away, colonies “shrinking,” and — if you look at night with a flashlight — a “polyp” that slowly crawls. Blow the colony with a baster and watch for something that moves.

Treatment: Dip zoas on intake (Coral Rx or similar) and inspect. Repeat dips every 4–8 days to catch hatchlings. Manual removal under magnification works for small colonies.

Zoanthid-Eating Sea Spiders

What to look for: Zoa-eating sea spider — tiny long-legged arthropod clinging at the mouth of a zoanthid polyp

How to ID them: Minuscule long-legged “spiders” that latch onto the polyp at its mouth and lay eggs inside it. They hide beneath the zoa’s mucus layer, which shields them from many dips. Polyps stay closed, look irritated, and die back. You’ll spot the spiders as thin, jointed legs poking from the base of closed polyps.

Treatment: Because the mucus layer protects them, a firm dip plus manual removal and a good rinse/agitation is most effective — swish the colony vigorously in the dip. Repeat, and inspect closely between rounds.

Sundial Snails

What to look for: Sundial snail (Heliacus) — small flat spiral shell with a checkered pattern, near a zoanthid colony

How to ID them: Heliacus snails have a small, flattened, spiral shell with a striking checkerboard/keeled pattern. They’re nocturnal zoanthid and palythoa predators — you’ll find them at the base of colonies or on the rock nearby after lights-out. Because they reproduce sexually, big infestations are rare, so hand-removal usually solves it.

Treatment: Manual removal at night is the simplest fix — pick them off with tweezers. Dip new zoa/paly colonies to avoid importing them.

The LPS & general coral threats

Coral-Eating (“Nuisance”) Limpets

How to ID them: Small conical “hat” shells that graze the rock. Most limpets are harmless algae-eaters — but some, and even the herbivores when underfed, will rasp the tissue off SPS bases and encrusting corals, leaving clean grazed tracks in the coral tissue. If you see a limpet parked on bare skeleton at the edge of receding tissue, it’s guilty.

Treatment: Manual removal. They’re easy to pick off; just relocate or remove any found actively on coral.

Fireworms (the bad bristleworms)

What to look for: Bearded fireworm vs. common bristleworm — fireworm is flatter, wider, with prominent white bristle tufts

The nuance: The great majority of bristleworms in your rock are beneficial detritivores — they eat leftover food and dead tissue and should be left alone. The exception is the bearded fireworm, a predatory species that will attack coral and even fish.

How to tell them apart: A common bristleworm is thin, reddish, and shy. A fireworm is flatter and wider, often larger, with dense, prominent white bristle tufts (“beard”) along its sides, and it doesn’t flee as readily. Never handle either bare-handed — the bristles sting and break off in skin.

Treatment: Only remove confirmed fireworms. Trap them or remove by hand with tweezers (wear gloves). A melanurus wrasse or arrow crab will hunt them. Leave your regular bristleworms alone — they’re a cleanup asset.

The one habit that prevents almost all of this: dip & quarantine

Every pest above shares a weakness — it hitchhikes in on a new coral. If nothing enters your display without being dipped and (ideally) quarantined first, you will almost never fight an outbreak. Our full walkthrough is here: how to dip corals and why you should never skip it. The short version:

Dip every incoming coral. Use a commercial coral dip (Coral Rx, Bayer-based recipes, or a povidone/iodine dip), swish and baste the coral to blow pests off, then rinse in clean tank water before placing. Inspect the dish — pests fall to the bottom. Cut off and discard bad-looking base plugs, where flatworm and nudibranch eggs hide.

Quarantine when you can. A simple observation tank for 2–6 weeks lets egg-based pests (AEFW, monti nudis) hatch where you can dip them again, before they ever reach your display. Pair this with a proper fish quarantine routine and your whole system stays clean.

Keep natural predators. Wrasses (six-line, melanurus, leopard, fairy), certain gobies, and a healthy pod population all suppress small pests between dips — one reason a thriving copepod population is a sign of a stable reef.

When to ask for help

If a coral is receding and you can’t match it to anything here, don’t guess and dose your whole tank — pull the coral, dip it, and photograph what comes off. Pest ID from a clear macro photo is fast, and the wrong treatment (a system-wide dose aimed at the wrong pest) can do more harm than the pest. Every coral that leaves our Key West facility is dipped and inspected before it ships, and we’re always happy to help you ID something — send a photo anytime.

A note on the photos in this guide: the captions describe exactly what to look for. We’re adding our own reference macro shots of each pest from the shop — if you have a clear photo of something you’re fighting, send it over and we’ll help you ID it (and may feature it here).

Dipping and quarantine only work if you also know what you are buying into. We wrote a seller-by-seller comparison of how 16 online coral shops describe their conditioning, what their live arrival guarantees actually cover, and which ones make pest claims nobody can back up: Where to Buy Coral Online.

Related. Most pest outbreaks arrive on a coral that nobody held long enough to see what hatched. We wrote up where that happens and why the cheap frag is rarely the cheap total: why the cheapest coral costs the most.

Keep a record of this coral

If you are working out what is on your coral, Coral Passport takes a photo and offers possible matches. It is not a guaranteed identification. It then keeps a personal record of that coral on your device: what you have, when you got it, and what you have watched it do.

Open Coral Passport