Aiptasia (glass anemones)

Berghia nudibranch

Peppermint shrimp

Copperband butterflyfish

Aiptasia-eating filefish / matted filefish

Klein's butterflyfish

Fast-cloning anemones with a strong sting that overgrow and burn neighboring corals and spread through the whole system.

Majano anemones

Emperor angelfish

Majestic angelfish

Blueface angelfish

Raccoon butterflyfish

Pretty, fast-spreading pest anemones with a potent sting that colonize rockwork and overgrow corals, and aiptasia remedies mostly fail on them.

Zoa-eating nudibranchs

Melanurus wrasse

Six-line wrasse

Yellow coris wrasse

Specialist zoanthid feeders that hide by day, chew polyps at night and lay durable egg ribbons on the colony, so one hitchhiker becomes a colony-killing population.

Montipora-eating nudibranchs

Obligate Montipora feeders that hide on colony undersides and reproduce in place, quietly stripping tissue from every Montipora in the system.

Acropora-eating flatworms (AEFW)

Target / spotted mandarin

Nearly invisible Acropora parasites that feed and lay eggs on the coral for weeks before damage shows, then strip colonies base-up.

Red bugs

Dragonface pipefish

Tiny parasitic copepods that irritate Acropora, costing color, polyp extension and growth; colonies usually decline slowly rather than dying overnight.

Black bugs

Microscopic parasitic copepods that burrow into Acropora polyps, are harder to kill than red bugs, and keep spreading on traded frags because nobody can see them.

Polyclad flatworms (clam / Euphyllia eaters)

Big nocturnal predators that can empty a Tridacna clam overnight, wipe out snail crews and steadily eat Euphyllia heads while hiding in rock all day.

Digitate hydroids

Halloween hermit crab

Lynx / fringeback nudibranchs

Stinging single polyps that emerge at night and can multiply into hundreds, holding zoanthids closed and irritating corals and small fish.

Colonial hydroids

Keyhole limpets

Bumblebee snail

Encrusting colonies rooted into the rock that sting and smother corals, shrug off nutrient control, and regrow from fragments after casual removal.

Pest sponges — boring sponge

Urchins

White-spotted doris nudibranch

Boring sponges tunnel inside rock, coral skeleton and clam shells, hollowing them from within so damage is advanced before the surface shows much.

Vermetid snails

Emerald crab

Sessile tube snails multiply across rockwork and their mucus feeding nets irritate nearby corals and slow their growth.

Sundial snails

Specialist zoanthid predators that hollow out polyps from inside the colony and can strip a colony over days to weeks.

Pyramidellid snails (pyramid snails)

Sixline wrasse

Eight-line wrasse

Green wrasse

Blood- and tissue-feeding parasites of giant clams that gather at the mantle edge at night and can kill a clam when numbers explode.

Eunicid worms (bobbit worms)

Large predatory species ambush small fish and invertebrates at night and can take chunks out of corals; a big specimen can work through livestock for months before it is seen.

Fireworms (bearded fireworm)

Melanurus and other Halichoeres wrasses

A genuine coral predator that feeds on stony corals, gorgonians and anemones, and its hollow venomous bristles deliver a painful sting to bare hands.

Asterina starfish

Harlequin shrimp

Most populations graze algae and biofilm harmlessly for years, while a minority of reports involve tissue grazing on zoanthids and SPS once numbers get very high.

Cirolanid isopods

Parasitic species swim out at night, latch onto sleeping fish and feed on blood and flesh; repeated attacks weaken and can kill fish one by one, and no reef-safe cure exists.

Gorilla crabs / xanthid crabs

Opportunistic omnivores that grow bolder with size, picking at coral polyps, snails and shrimp, and capable of catching small sleeping fish.

Mantis shrimp (hitchhiker)

A burrow-dwelling predator that smashes or spears snails, hermits, shrimp and small fish; most keepers remove it from a community reef and many rehome it to a species tank where it is prized.

Coral-boring spionid worms

The worms tunnel through living coral skeleton, weakening the colony, deforming growth and stunting or slowly killing SPS and LPS from inside.

Gall crabs (coral gall crabs)

Obligate coral dwellers that live in a pit or gall in the skeleton and feed on coral mucus and trapped particles; many hosts grow around them unharmed, while some corals show chronic irritation and decline.

Bubble algae

Pitho crab

Foxface rabbitfish

Sally Lightfoot crab

Spreads bubble by bubble across rock and frag plugs and can crowd out corals, while a small patch is easy to lift out whole.

Bryopsis

Sea hare

Some mithrax crabs and mollusks

Roots into rock with holdfasts, resists most grazers, and can overgrow and shade out corals while manual pulling alone rarely finishes it.

Dinoflagellates (problem dinos)

Toxic types (notably Ostreopsis) can kill snails and stress fish and corals, and blooms coat sand and rock fast in low-nutrient tanks.

Cyanobacteria (red slime)

Trochus snails

Cerith snails

Some hermit crabs

Slimy mats spread over sand, rock and coral bases and can smother tissue, and blooms signal a flow and nutrient imbalance worth fixing.

Green hair algae (GHA complex)

Turbo snails

Tangs

Lawnmower blenny

Emerald crabs

Pincushion urchin

Long filament mats smother coral bases and trap detritus, and a bloom can cover rockwork in weeks while nutrients stay available.

Diatoms (new-tank brown film)

Astrea snails

Nerite snails

A normal new-tank phase that feeds microfauna and fades as silicate is used up and the tank matures; heavy films can be siphoned for looks.

Red planaria flatworms

Lunare wrasse

Springer damsel

Populations explode and blanket corals, blocking light, and a mass die-off releases toxins that can kill fish and corals.

Stomatella Snails

Herbivorous algae grazers that never bother corals, fish, or other invertebrates.

Collonista Snails

Tiny herbivorous grazers, often called mini turbos, that feed on algae film and never eat zoanthids.

Common Bristleworms

Scavenging detritivores with no jaws; they eat leftovers and dead matter and cannot kill healthy livestock or corals.

Spaghetti Worms

Tube-dwelling detritivores that collect fine organic particles and have no sting and no interest in corals, clams, or fish.

Peanut Worms

Unsegmented deposit and suspension feeders that live inside rock and eat detritus; they have no jaws, bristles, or sting.

Mini Brittle Stars

Tiny detritus-eating brittle stars that stay around 2 cm across and clean crevices no other cleanup animal can reach.

Amphipods and Copepods

Beneficial micro-crustaceans that eat detritus and algae and serve as live food for fish and corals; they never parasitise fish.

Spirorbid Worms

Sessile filter feeders in tiny calcareous coils that strain particles from the water and harm nothing in the tank.

Pineapple Sponges

Small calcareous filter-feeding sponges that pull particles and dissolved waste from the water and never sting, spread over corals, or prey on anything.

Chitons

Slow, herbivorous molluscs that graze filamentous and film algae from rock and glass and ignore corals and livestock.

Limpets

Common limpets are harmless algae grazers, but keyhole limpets can occasionally graze soft or large-polyp stony coral tissue when other food runs short, so they earn watching rather than blind trust.

Pests