Algae IDs

Chrysophyte-type golden films — microscope confirmation required

Identify the cells before choosing treatment.

Pest algae · Microscopic ID · Reef-tank appearance

Quick ID

Yellow-gold to brown, wispy or gelatinous coatings on rock and sand. Small golden cells may sit within mucus; this appearance is shared by several unrelated organisms.

Benefits / uses

Golden algae are part of aquatic food webs; dense coatings are undesirable around coral.

Problems / risks

Thick growth can cover coral and trap debris. “Golden algae” is a broad common name, not a reliable species identification.

Will anything eat it?

Grazing is inconsistent. Confirm that the film is not a potentially toxic dinoflagellate before adding animals.

Products / equipment

Some golden algae form silica structures, but not every reef golden film is driven by silicate. Use silicate-removal media only for a demonstrated issue and monitor phosphate too.

Common lookalikes

Diatoms, dinoflagellates, bacterial films and haptophytes. Chrysophytes are not diatoms; Prymnesium “golden algae” is a different group.

Care / control

Siphon and remove loose coatings, maintain balanced nutrients and inspect RO/DI water and resin. Investigate silicate inputs if measured high; they are not proof of the cause.

Oedogonium spp.

Primarily freshwater; do not use this name for ordinary marine hair algae.

Freshwater · Slime & lookalikes

Quick ID

Unbranched green filaments; microscope-visible cap rings are an important identifying feature.

Benefits / uses

A freshwater primary producer, rather than a usual marine refugium crop.

Problems / risks

Can become nuisance filamentous growth in freshwater planted systems.

Will anything eat it?

Freshwater grazing advice cannot be transferred directly to a reef aquarium.

Products / equipment

Do not select a marine algicide from a freshwater algae name; confirm the organism and system first.

Common lookalikes

Other unbranched green filaments, especially marine hair algae identified only from a photo.

Care / control

In a marine tank, recheck the ID. In freshwater, remove growth and review planted-tank conditions.

Benthic diatoms; Bacillariophyta

Common in new tanks; a persistent bloom deserves a source check.

Microalgae · New-tank growth

Quick ID

Fine golden-brown dust on sand, rock or glass that wipes or siphons away.

Benefits / uses

Food for small grazers and part of a developing reef food web.

Problems / risks

Heavy coatings spoil appearance and may cover nearby coral surfaces.

Will anything eat it?

Trochus, cerith and nerite snails commonly graze films; fish vary.

Products / equipment

RO/DI maintenance first. RowaPhos removes silicate and phosphate; use only when indicated and monitor phosphate.

Common lookalikes

Cyano sheets and dino mucus; microscopy is useful when brown coatings persist.

Care / control

Siphon or wipe, allow new systems to mature and check source-water quality for persistent blooms.

Nannochloropsis, Isochrysis, Tetraselmis and other cultured microalgae

Useful live food when the culture and amount suit your animals.

Microalgae · Live food

Quick ID

Individual cells are microscopic; a culture may look green, golden or brown.

Benefits / uses

Feeds copepods and suitable filter feeders; different cell sizes suit different consumers.

Problems / risks

Overfeeding or a failing culture can add unwanted organic material.

Will anything eat it?

Consumed by appropriate plankton feeders and pods; algae-grazing fish are not the main users.

Products / equipment

Live marine blends such as AlgaeBarn OceanMagik are feeds, not guaranteed outbreak cures.

Common lookalikes

An accidental green-water bloom is not evidence of a clean, suitable food culture.

Care / control

Choose clean cultured marine strains, store as directed and start with modest feeding.

Chaetomorpha spp.

A popular contained refugium crop for nutrient export and pod habitat.

Refugium algae · Green macroalgae

Quick ID

Tangled, springy green spaghetti-like strands; no leaves or creeping runner.

Benefits / uses

Growth uses dissolved nutrients; harvesting physically exports the captured material.

Problems / risks

Dense or dirty masses shade themselves, trap debris and can die back.

Will anything eat it?

Not the most palatable fish food; keep the crop protected in a refugium.

Products / equipment

Brightwell ChaetoGro supplies trace nutrients when needed. It contains no nitrate/phosphate and is not an algicide.

Common lookalikes

Hair algae is usually softer and attached; mixed masses may contain nuisance algae.

Care / control

Provide suitable light and flow, thin regularly and remove decaying strands.

Ulva spp.

A fast-growing refugium crop and edible macroalga; contain it deliberately.

Refugium algae · Display algae · Fish food

Quick ID

Thin green sheets, ruffles or narrow leafy blades; form varies by species.

Benefits / uses

Harvestable nutrient export, pod habitat and fresh herbivore food.

Problems / risks

Loose fragments can spread; grazing can strip a display planting.

Will anything eat it?

Many tangs and rabbitfish readily eat it; individual appetite still varies.

Products / equipment

Clean cultured Ulva is widely sold; appropriate lighting and harvesting matter more than an additive.

Common lookalikes

Blade Caulerpa has thicker fronds linked by runners; film algae forms a coating.

Care / control

Grow in a protected refugium, harvest regularly and remove loose or decaying pieces.

Gracilaria spp.; including long ogo forms

Useful food and ornamental growth; species differ in texture and grazing appeal.

Refugium algae · Display algae · Fish food

Quick ID

Red, burgundy or brownish branching stems; many ogo forms are fine and bushy.

Benefits / uses

Nutrient export when harvested, pod shelter and edible seaweed for herbivores.

Problems / risks

Can collect debris; loose growth needs containment and adequate circulation.

Will anything eat it?

Many tangs and rabbitfish eat soft ogo. Tougher pom-pom forms can be less readily grazed.

Products / equipment

Clean cultured red ogo is commercially sold; no chemical control needed when deliberately grown.

Common lookalikes

Pom-pom Gracilaria hayi has tougher, broader branches; Halymenia has flatter, softer blades.

Care / control

Keep moving water through the crop, prune and remove declining growth.

Caulerpa racemosa complex

Useful in a controlled crop, but invasive when it escapes into rockwork.

Refugium algae · Pest if escaped · Green macroalgae

Quick ID

Green bead-like branchlets attached to stems and creeping runners.

Benefits / uses

Fast growth can provide harvestable nutrient export and habitat.

Problems / risks

Runners spread into rock; reproductive events or die-back can foul the water.

Will anything eat it?

Some herbivorous fish eat it; palatability varies and runners can remain.

Products / equipment

Manual removal for escaped growth. No selective reef-safe chemical recommended; check local availability restrictions.

Common lookalikes

Green bubble algae has vesicles fixed to rock without a Caulerpa runner.

Care / control

Keep contained, prune the whole runner and remove whitening or dying growth.

Caulerpa prolifera

An attractive managed macroalga; keep runners away from coral rock.

Refugium algae · Display algae · Green macroalgae

Quick ID

Broad oval or strap-like green fronds joined by creeping runners.

Benefits / uses

Harvestable nutrient export and planted marine habitat.

Problems / risks

Spreads by runners; reproductive events and declining growth need monitoring.

Will anything eat it?

Some tangs and rabbitfish may graze it; do not count on fish for containment.

Products / equipment

Sold as cultured macroalgae. Physical containment first; check restrictions and never release aquarium algae outdoors.

Common lookalikes

Ulva has thin sheets without runners; seagrasses are flowering plants rather than algae.

Care / control

Grow in a contained area, prune runners and remove unhealthy material promptly.

Halymenia spp.; trade names vary

A colourful ornamental macroalga; verify the species sold under the trade name.

Display algae · Refugium algae · Red macroalgae

Quick ID

Soft red blades or fronds, often with orange-looking tips under reef lighting.

Benefits / uses

Colour, shelter and some nutrient uptake; harvest for actual export.

Problems / risks

Can tear, decline or be eaten; slower ornamental growth is not a guaranteed nutrient solution.

Will anything eat it?

Some herbivorous fish eat it, so protect valuable specimens from grazers.

Products / equipment

Clean cultured Halymenia is sold commercially; growth support rather than algicide is appropriate.

Common lookalikes

Gracilaria usually has more stem-like branches; several Halymenia species share the same nickname.

Care / control

Provide suitable light and gentle circulation; secure without crushing and prune declining tissue.

Halimeda spp.

Usually welcome ornamental growth; it has calcium and alkalinity demands.

Display algae · Calcified green macroalgae

Quick ID

Joined green coin, fan or cactus-like segments that feel firm and calcified.

Benefits / uses

Distinctive display structure; contributes calcareous material as old segments break down.

Problems / risks

Growth consumes calcium and alkalinity; die-back should be removed.

Will anything eat it?

Often less readily eaten than soft seaweed; grazing varies.

Products / equipment

Normal tested reef calcium/alkalinity supplementation if required; no algicide for healthy desired growth.

Common lookalikes

Soft Ulva and blade Caulerpa are not calcified chains of segments.

Care / control

Monitor calcium and alkalinity, allow space and remove white or decaying growth.

Calcifying red algae; several coralline groups

Usually desirable on rock; unwelcome on viewing panels and equipment.

Display algae · Calcified red algae

Quick ID

Hard pink, purple, red or pale crusts that need scraping rather than wiping.

Benefits / uses

Natural reef appearance and a firmly occupied surface on live rock.

Problems / risks

Adds calcium/alkalinity demand; can obstruct pumps and cover glass.

Will anything eat it?

Some urchins and specialist grazers eat it; most fish will not remove hard crusts.

Products / equipment

Test-guided calcium/alkalinity support. Live coralline inoculants are optional; they cannot replace suitable conditions.

Common lookalikes

Red cyano wipes or peels away; purple colour alone does not prove coralline.

Care / control

Keep reef chemistry stable; scrape viewing panels and clean equipment as needed.