Algae IDs

Bryopsis spp.

An invasive feathered alga; ordinary grazers often leave it.

Pest algae · Green macroalgae

Quick ID

Small green feathers: a central stalk with fine side branches.

Benefits / uses

Photosynthesises, but offers little benefit in a coral display.

Problems / risks

Anchors firmly and regrows from remaining growth; can crowd coral bases.

Will anything eat it?

Fish and snails often ignore it; grazing alone is unreliable.

Products / equipment

Flux Rx (fluconazole) is labelled for Bryopsis. Manufacturer: do not use with macroalgae or a refugium.

Common lookalikes

Hair algae lacks neat feathers; feather Caulerpa has larger fronds and creeping runners.

Care / control

Isolate affected plugs where practical; remove growth and siphon fragments.

Several filamentous green algae; including Derbesia-type forms

Useful grazing food in small amounts; a nuisance when it blankets the display.

Pest algae · Green macroalgae

Quick ID

Soft green threads or tufts, rather than tidy feather-shaped branches.

Benefits / uses

Grazing food; intentionally grown algae can export nutrients when harvested.

Problems / risks

Long growth catches debris, shades coral and can return after pulling.

Will anything eat it?

Many tangs, rabbitfish, blennies and snails eat short growth; appetite varies.

Products / equipment

Flux Rx is labelled for green hair algae; avoid use with macroalgae/refugia. Removal and husbandry come first.

Common lookalikes

Bryopsis, coarse turf and freshwater Oedogonium; a photo rarely establishes the species.

Care / control

Pull and siphon, clean detritus traps and review feeding, light and nutrient export.

Mixed turf communities; Cladophora spp. are one example

A growth form rather than one species; established patches can be stubborn.

Pest algae · Green macroalgae

Quick ID

Short, dense, coarse or branching green growth attached tightly to rock.

Benefits / uses

Natural grazing habitat; turf is useful inside a managed algae scrubber.

Problems / risks

In the display it traps debris and can occupy space around coral bases.

Will anything eat it?

Some urchins and herbivorous fish graze it; coarse mature patches may resist grazing.

Products / equipment

Manual removal first; no dependable selective reef-safe product recommended.

Common lookalikes

Soft hair algae; microscope examination separates similar filamentous genera.

Care / control

Scrub accessible rock outside the display in saltwater; siphon loosened material.

Valonia / Ventricaria-type bubble algae

A few bubbles are manageable; expanding clusters become a nuisance.

Pest algae · Bubble algae

Quick ID

Glossy green spheres or oval vesicles attached to rock or plugs.

Benefits / uses

Interesting natural growth, but not a preferred nutrient-export crop.

Problems / risks

Clusters spread into crevices and compete for space around corals.

Will anything eat it?

Some emerald crabs and rabbitfish eat it; neither is a guaranteed solution.

Products / equipment

Manual removal first; no dependable selective reef-safe product recommended.

Common lookalikes

Air bubbles disappear when disturbed; red bubble algae belongs to different groups.

Care / control

Lift bubbles and attachment points intact where possible; siphon debris and revisit regrowth.

Dictyota spp.

A brown macroalga best contained before it spreads across coral rock.

Pest algae · Brown macroalgae

Quick ID

Thin, flat brown or olive ribbons that repeatedly split into forks.

Benefits / uses

Natural reef habitat; usually unwanted in a mixed coral display.

Problems / risks

Fragments and attached growth can establish new patches.

Will anything eat it?

Chemical defences deter some herbivores; grazing depends on species and individual animals.

Products / equipment

Manual removal first; no dependable selective reef-safe product recommended.

Common lookalikes

Lobophora makes overlapping plates; Halymenia is usually red and softer.

Care / control

Remove occupied plugs or accessible growth; collect fragments rather than dispersing them.

Lobophora spp.

Leathery brown growth can overgrow or compete with coral.

Pest algae · Brown macroalgae

Quick ID

Brown, overlapping fan-shaped blades or low plates with a leathery feel.

Benefits / uses

Part of natural reef communities; no special advantage in a coral display.

Problems / risks

Can occupy coral margins; effects vary between species and conditions.

Will anything eat it?

Some rabbitfish and urchins graze it, while other herbivores avoid it.

Products / equipment

Manual removal first; no dependable selective reef-safe product recommended.

Common lookalikes

Brown coralline is hard and calcified; Dictyota forms narrow forked ribbons.

Care / control

Remove accessible plates and attached growth; monitor coral margins and regrowth.

Cyanobacteria; photosynthetic bacteria

An algae lookalike; treatment differs from true macroalgae.

Pest algae · Slime & lookalikes · Not a true alga

Quick ID

Slippery red, purple, green or dark sheets; sometimes with trapped bubbles.

Benefits / uses

Natural microbes, but thick aquarium mats are undesirable.

Problems / risks

Mats cover sand and coral surfaces; large die-offs can affect oxygen.

Will anything eat it?

Most fish and common algae snails are unreliable controls.

Products / equipment

Boyd Chemiclean is marketed for cyano-associated stains. Follow the current label and provide strong aeration; Dino X does not treat cyano.

Common lookalikes

Diatoms form dust; dinos can form stringy mucus. Colour and bubbles alone do not confirm an ID.

Care / control

Siphon mats, remove trapped detritus and improve circulation and nutrient balance.

Benthic dinoflagellates; several different genera

Confirm the group under a microscope before choosing treatment.

Pest algae · Microalgae · Slime & lookalikes

Quick ID

Brown mucus, strings or bubbles may occur; these signs are not diagnostic.

Benefits / uses

Many dinoflagellates are beneficial; this card concerns nuisance surface blooms.

Problems / risks

Some nuisance groups produce toxins and harm grazers or coral.

Will anything eat it?

Do not rely on fish or snails; some groups are hazardous to grazers.

Products / equipment

Fauna Marin Dino X is an option after ID. Its manual warns about macroalgae, echinoderms, sea hares, clams and sponges.

Common lookalikes

Cyano, diatoms and golden algae; similar brown coatings need different approaches.

Care / control

Obtain a microscope ID, siphon growth and avoid driving nitrate/phosphate to zero. UV helps some water-column groups.

Amphidinium spp. — small-cell hobby grouping

Identify the cells before choosing treatment.

Pest algae · Microscopic ID · Reef-tank appearance

Quick ID

Tiny golden-brown oval cells, often moving rapidly; use a calibrated scale and video, not magnification alone. In the tank: a persistent brown film on sand, glass or rock.

Benefits / uses

Part of microbial communities; a dense display bloom offers no practical benefit.

Problems / risks

Can irritate coral and recur after cleaning. Small-cell is a hobby grouping, not a confirmed species name.

Will anything eat it?

Grazing is variable; extra snails are not a dependable cure and may be exposed to harmful cells.

Products / equipment

A correctly sized enclosed UV steriliser is an option for swimming cells. Nutrient supplements only when testing shows a shortfall; follow labels.

Common lookalikes

Large-cell Amphidinium, other small flagellates and diatoms. Film colour alone cannot distinguish them.

Care / control

Confirm the ID, siphon deposits and maintain stable, measurable nitrate and phosphate. Check a night sample: UV can help when cells enter the water.

Amphidinium spp. — large-cell hobby grouping

Identify the cells before choosing treatment.

Pest algae · Microscopic ID · Reef-tank appearance

Quick ID

Larger, flattened oval cells with a small off-centre cap or beak; often glide over the slide. In the tank: rust-brown sand patches, sometimes with little mucus or bubbling.

Benefits / uses

Normal low-level microbial diversity; an extensive sandbed bloom is undesirable.

Problems / risks

Often persists within the sand. Some outbreaks appear less toxic, but this does not establish that a strain is harmless.

Will anything eat it?

Some micrograzers may consume cells; do not buy sand-sifting livestock as a guaranteed treatment.

Products / equipment

Silicate dosing to encourage competing diatoms is an optional, monitored advanced approach after confirmed ID—not a universal cure. Avoid blind dosing.

Common lookalikes

Diatoms, small-cell Amphidinium and mixed films; appearance and apparent cell size need context.

Care / control

Confirm under a microscope, remove surface deposits gently and correct nutrient depletion. UV often has limited reach because much of the population stays in the sand.

Ostreopsis spp.

Identify the cells before choosing treatment.

Pest algae · Microscopic ID · Reef-tank appearance

Quick ID

Flattened oval or teardrop cells, often pivoting around an attachment point. In the tank: brown mucus and long bubble-bearing strings on rock, glass, sand or coral.

Benefits / uses

A natural benthic organism; dense nuisance blooms are best controlled.

Problems / risks

Some species produce potent toxins and can harm grazers. Avoid splashing or aerosolising suspect growth; do not scrub or pressure-wash affected rock into the air.

Will anything eat it?

Do not add snails or herbivorous fish as a remedy for a potentially toxic bloom.

Products / equipment

UV and activated carbon are supportive tools. Chemical treatment requires checking the current label and livestock compatibility; no guaranteed one-bottle cure.

Common lookalikes

Coolia, Prorocentrum, chrysophytes and cyano; bubbles and strings alone are not a diagnosis.

Care / control

Microscope confirmation first. Siphon gently, maintain aeration and balanced nutrients. Enclosed UV can help when cells detach into the water; use activated carbon according to its label.

Coolia spp.

Identify the cells before choosing treatment.

Pest algae · Microscopic ID · Reef-tank appearance

Quick ID

Rounded, lens-like cells with visible grooves; the view changes as the cell rotates. In the tank: brown films or patches on sand and rock, sometimes mixed with other growth.

Benefits / uses

A natural reef microbe; a dense nuisance bloom has no display benefit.

Problems / risks

Toxicity varies by species and strain. A round cell does not establish the species or its toxicity.

Will anything eat it?

No dependable clean-up crew solution; avoid exposing new grazers to an unidentified bloom.

Products / equipment

Enclosed UV is useful only for cells passing through it; carbon may support toxin management. Avoid stacking chemical treatments.

Common lookalikes

Ostreopsis and Gambierdiscus; specialist morphology or molecular work may be needed for an exact species ID.

Care / control

Record a microscope video and seek confirmation. Siphon patches and stabilise nutrients. Check whether cells leave surfaces at night before relying on UV.