Aquarium Algae Identification: Every Type, Cause, and How to Fix It

Most algae problems fail because the wrong type gets treated. A blackout clears green water but does nothing for black beard algae. Nerite snails polish glass spots but cannot touch hair algae on moss. Correct identification tells you which lever to pull - light, nutrients, flow, or carbon - and saves weeks of guesswork.

Why identification comes first

Algae is not one organism. It is a loose group of photosynthetic life - green algae, red algae, diatoms, and even bacteria (cyanobacteria) that look like algae. Each type responds to different conditions. Hair algae often tracks excess light and nutrient imbalance. Black beard algae (BBA) usually signals unstable CO₂. Diatoms appear in new tanks and fade on their own. Treating every bloom the same way is why so many fixes “work for a week” and come back.

Start with what you see and feel, confirm with the decision tree, then read the dedicated guide for that type. Log your water parameters while you work - patterns in nitrate, phosphate, and light hours often explain why a particular algae won out.

Four field tests (before you treat anything)

You do not need a microscope. Run these quick checks at the tank:

1. Touch test: Slimy and sheets peel away? Likely cyanobacteria. Fuzzy but not slimy? Hair, fuzz, or staghorn. Hard and will not rub off? Green spot or BBA.
2. Wipe test: Wipes clean with a finger but returns overnight? Often diatoms or fuzz in a young tank. Will not wipe at all? Green spot or BBA on glass.
3. Smell test: Earthy or rotten-egg smell when disturbed? Strong sign of cyanobacteria - not true algae.
4. Water-column test: Can you see through the tank at all? If the water itself is green, that is green water (free-floating algae), not a surface coating.

Algae vs things that look like algae

What you see	Probably is	Key difference
Teal/green slimy sheets on substrate	Cyanobacteria	Bacteria, not algae; smells when disturbed; bubbles under sheets
Brown dust on glass and plants (tank under 8 weeks)	Diatoms	Wipes off easily; common during cycling; fades as tank matures
Short fluffy coating on new leaves	Fuzz algae / biofilm	Normal in tanks under 6 months; shrimp graze it down
White cottony patches on wood or injured fish	Fungus / saprolegnia	Not algae - see fish diseases if on fish
Pea-soup green water	Green water	Algae suspended in the water column, not on surfaces

The severity scale (1-4)

Every type in the table below is rated 1-4. Use this to decide urgency:

• 1/4 - Cosmetic: Normal in new tanks (diatoms). Monitor, scrape, wait. No livestock changes needed.
• 2/4 - Manageable: Annoying but reversible with parameter and light adjustments (fuzz, hair, green spot).
• 3/4 - Persistent: Needs a deliberate treatment plan and cause correction (BBA, staghorn, cladophora).
• 4/4 - Tank-wide: Can smother plants or cloud the whole tank (cyanobacteria, severe green water).

What actually feeds algae

Algae needs light and nutrients. In planted tanks, CO₂ availability often decides which algae wins when those are out of balance:

• Light: Too many hours or too intense for your plant mass is the most common trigger. Start at 6-8 hours and increase only when plants demand it.
• Nutrients: Excess nitrate and phosphate from overfeeding fuel blooms. But some algae (green spot, BBA) also appear when nutrients are too low for plants - starving plants lose the competition.
• Flow: Dead zones collect debris and favour cyanobacteria and BBA on slow-moving surfaces.
• CO₂ (planted tanks): Unstable injection - on/off swings, end-of-tank drop - is a recurring BBA and staghorn trigger. Stability matters more than peak ppm.
• New tank biology: An uncycled or freshly cycled tank lacks mature plant and bacterial competition. Brown diatoms and fuzz are common until the system settles (see how to cycle a fish tank).

Fix order: correct light and nutrients first, then flow and CO₂, then manual removal, then biological helpers or spot treatments. Skipping straight to algae eaters treats the symptom, not the cause.

All types at a glance

Type	Severity	Primary cause sketch	First-line fix	Biology helpers
Blue-green / cyanobacteria	4/4	Low-nitrate + stagnant flow combos	Blackout + flow reprogramming	Limited - manual dominates
Green water	3-4/4	Photon overdrive + nitrogen spurts	UV steriliser sizing	Temporary Daphnia pulses
Black beard (BBA)	3/4	CO2 choreography drift	Stabilise carbon + prudent spot dosing	True Siamese algae eater (verify ID)
Staghorn	3/4	CO2 jitter + micronutrient starvation	Prune / targeted oxidative spot	Shrimp lawns post-treatment
Hair algae	2-3/4	Photon macros vs dissolved carbon tug-of-war	Harvest + photoperiod diet	Adult Amanos, cautious flagfish
Green spot	2/4	Chronically depressed phosphate arenas	Raise PO4 deliberately + Nerites	Nerite snails (glass rims)
Fuzz / young-tank coatings	2/4	Immature biofilm choreography	Shrimps + time + CO2 mastery	Neo / cherry shrimp scouting parties
Diatoms (brown dust)	1/4	Silica residency window	Patience & scraping	Otocinclus once stable
Cladophora	3/4	Hitchhiking hardscape clones	Hardscape oxidative dip + brute removal	Limited once entrenched deep

Decision tree (start here)

Work top to bottom. First match wins.

1. Is the water itself green/pea-soup coloured? → Green water (not a surface problem).
2. Slimy teal or blue-green sheets that smell when poked, often on substrate? → Cyanobacteria (bacteria, not true algae).
3. Dark bushy tufts on leaf edges, wood, or filter outflow - tough to pull off? → Black beard algae (BBA).
4. Branching, antler-like grey-green filaments that turn pink/red with hydrogen peroxide? → Staghorn algae.
5. Long green threads wrapping plants and moss? → Hair algae.
6. Hard green dots on glass that will not scrape off with a fingernail? → Green spot algae.
7. Short fluffy coating on new plants, tank under 6 months? → Fuzz algae (usually self-limiting).
8. Brown dusty film on glass/plants in a tank under 8 weeks? → Diatoms (normal during cycling).
9. Rough, steel-wool texture on rock or moss - fragments spread when pulled? → Cladophora.

Common misidentifications

• BBA vs staghorn: BBA forms compact dark tufts on edges; staghorn branches like deer antlers and often grows in clumps on slower-growing plants.
• Hair algae vs cladophora: Hair is soft and stringy; cladophora is stiff, rough, and snaps rather than stretches.
• Diatoms vs brown cyanobacteria: Diatoms wipe off as fine dust; cyanobacteria peels in sheets and smells.
• Green spot vs green dust: Spot algae forms hard individual dots; green dust is a soft film on glass that returns quickly - different causes and fixes.
• Fuzz vs BBA: Fuzz is light, short, and common on new growth; BBA is dark, bushy, and grips hard.

After you identify: fix in this order

1. Log parameters - ammonia, nitrite, nitrate, and (if planted) phosphate. One week of readings beats one guess.
2. Adjust light - reduce photoperiod or intensity before anything else.
3. Fix feeding and maintenance - less food, more water changes, siphon detritus from dead zones.
4. Correct CO₂ and flow (planted tanks) - stability over spikes.
5. Manual removal - scrape, trim affected leaves, export what you remove with a water change.
6. Biological helpers or spot treatment - only after steps 1-5. See individual guides for species-specific options.

Related guides

• Aquarium algae: causes and general fixes
• How to cycle a fish tank
• Aquarium water parameters
• Best algae eaters (and what they actually eat)
• Browse all freshwater guides