Knowledge Centre · Water quality
Algae in aquariums: types, causes and how to control it
"Algae" covers several genuinely different organisms — green spot, green dust, hair or string algae, black beard algae and brown diatoms all look, spread and respond to treatment differently, and none of them are the free-floating green water or the bacterial "blue-green algae" (cyanobacteria) covered in our other guides. Almost all of it is driven by the same two things — light and available nutrients — and a small amount is normal rather than a sign your tank is unhealthy. The sections below help you identify what you're actually looking at and bring it under control without over-reacting.
At a glance
What it usually means
Algae growth almost always tracks light exposure and available nutrients (mainly nitrate and phosphate), not "dirty water" in a general sense. A small amount on glass or older leaves is normal, not a fault.
Why it matters
Heavy or spreading algae can smother plants and slow-growing decor, clog equipment, and outcompete plants for light and nutrients — more a tank-health and maintenance issue than a direct danger to most fish.
Sensible immediate response
Identify the type before treating it, check your lighting duration and intensity, remove what you can manually, and address nutrient inputs (feeding, water changes) rather than reaching for a product first.
What causes algae in an aquarium
Algae are plant-like organisms, and like plants they need light and nutrients to grow — mainly nitrate and phosphate, which build up from fish waste, uneaten food and decomposing organic matter. Phosphorus in particular is widely documented as the nutrient that most consistently limits algal growth in freshwater, meaning even a small excess of it, combined with enough light, can trigger a bloom[2]. That's why algae problems are so often really a lighting or nutrient-management problem, not a hygiene problem in the everyday sense — a visually "clean" tank with too much light or too many nutrients available will still grow algae, and a well-maintained tank with sensible light and nutrient levels usually won't, regardless of how often you scrub the glass.
A new tank is a common flashpoint for this. Before plants are established enough to compete for nutrients and before a routine is settled, light and nutrients are often out of balance, which is part of why brown diatom algae in particular is so strongly associated with tanks in their first weeks. See our guide to starting a new aquarium if that's the situation you're in.
Types of algae and how to identify them
Green spot algae
Small, hard, circular green dots stuck firmly to glass, slow-growing leaves or decor. It's tough to remove by wiping and usually needs scraping with a blade or pad. It's commonly linked to low CO2 or phosphate relative to light in planted tanks, or simply to glass that's rarely cleaned in a lower-light tank.
Green dust algae
A fine, even, powdery green film across the glass that wipes off easily but often returns within days, sometimes in a cycle where it clears and comes back roughly every couple of weeks. It's a distinct organism from green spot algae despite the similar colour, and is generally considered a light-driven, self-limiting nuisance rather than a sign of a deeper problem.
Hair, string or blanket algae
Long, thin green filaments, sometimes fine and hair-like, sometimes thicker and matted into clumps ("blanket algae"), draping over plants, decor or substrate. This is filamentous algae, one of three broad algae categories (alongside planktonic and macroalgae) commonly used to organise identification[1], and it's one of the most commonly reported nuisance types — it spreads quickly under excess light and nutrients and can smother slower-growing plants if left unchecked.
Black beard or brush algae (BBA)
Dark grey-black tufts, often described as looking like a beard or a stiff brush, typically attached to leaf edges, driftwood or slow-flow areas of the tank. It's notably more resistant to manual removal and to grazing by most algae-eating species than the other types here, and it's commonly linked to fluctuating or low CO2 alongside adequate light in planted tanks, rather than nutrient excess alone.
Brown algae (diatoms)
A soft, brown, dusty coating that wipes off easily, most strongly associated with new tanks in their first several weeks. It typically fades on its own as a tank matures and silicate levels (which diatoms rely on) are used up, and heavy scrubbing is rarely necessary — patience and routine glass cleaning are usually enough.
Cladophora
A coarse, dark-green, branching filamentous algae that forms dense, springy, wool-like clumps or tangled mats, distinctly firmer and coarser to the touch than fine hair algae rather than a variation on it. It attaches strongly to décor, substrate and slow-growing hardscape and is notably harder to fully remove by hand than hair or string algae, since fragments left behind can regrow. It's driven by the same light-and-nutrient combination as other filamentous algae, but its resilience makes prevention (catching it early, physically removing as much as possible while it's still a small patch) more valuable than with most other types here.
Staghorn algae
A reddish-brown to grey-green filamentous algae that branches in a distinctive antler-like pattern, usually attached to slow-growing plant leaves, decor or equipment, and notably tougher and more wiry than hair algae. It's one of the more resistant types to manual removal and to grazing by most algae-eating livestock, and is generally linked to the same excess-light-and-nutrient pattern as hair, string and blanket algae, sometimes worsened by unstable CO2 in a planted tank running injection.
Is some algae normal?
Yes. A light film on glass between cleans, a bit of green on older, slower-growing leaves, or a thin coating on décor in a well-lit tank is normal and doesn't indicate a problem on its own — it's part of what a healthy, mature tank's ecosystem is often growing anyway, and many shrimp, snails and algae-grazing fish rely on it as a genuine food source rather than a last resort. What's worth addressing is algae that's spreading quickly, smothering plants, clogging equipment, or returning heavily within days of being cleaned off, since that pattern points to light or nutrients being genuinely out of balance rather than normal background growth.
Quick reference: type, cause and what helps
Identification should come before treatment, and lighting and nutrients aren't identical explanations for every organism below — several of these are as much about CO2 stability, silicate, or simply being a new, still-settling piece of wood as they are about "too much light."
| Colour | Texture / how it comes away | Where it grows | Common lookalikes | Safest first response |
|---|---|---|---|---|
| Green | Small, hard dots; needs scraping, won't wipe off | Glass, slow-growing leaves, decor | Green dust algae (softer, wipes off) | Scrape; review light intensity/duration |
| Green | Fine, powdery film; wipes off easily, often returns | Glass | Green spot algae (hard, needs scraping) | Regular wiping; usually self-limiting |
| Green | Long, fine strands, loose clumps | Plants, decor, substrate | Cladophora (coarser); staghorn (branching, wirier) | Manual removal; reduce light duration; export nutrients via water changes |
| Dark green | Coarse, springy, wool-like clumps; strongly attached, fragments regrow | Décor, substrate, slow-growing hardscape | Hair/string algae (finer, less resilient) | Remove as much as possible while still a small patch; prevention matters more here |
| Reddish-brown to grey-green | Wiry, branching, antler-like; resistant to grazing and manual removal | Slow-growing leaves, decor, equipment | Black beard/brush algae (darker, tufted rather than branching) | Manual removal of affected material; stabilise CO2 in planted/injected tanks |
| Dark grey-black | Stiff tufts, brush- or beard-like | Leaf edges, driftwood, slow-flow areas | Staghorn (branching rather than tufted) | Manual removal; stabilise CO2 where relevant; most resistant to grazing |
| Soft brown | Dusty coating, wipes off easily | Glass, substrate, decor — very common in new tanks | None commonly confused with it | Routine glass cleaning; usually resolves as the tank matures |
| Blue-green, red or near-black | Slimy sheet, peels off in one connected piece; musty smell when disturbed | Substrate, decor, glass | Black beard algae, hair algae — but this is cyanobacteria, not true algae | See our dedicated cyanobacteria guide — algae-specific products often don't work on it |
| Green | Suspended through the entire water column, not attached to a surface | Whole tank (cloudy, tinted water) | Bacterial bloom cloudiness (whitish, not green) | This is green water, treated differently to surface-growing algae — see our cloudy water guide |
| White to greyish | Soft, sometimes faintly fuzzy film | New, untreated driftwood specifically | Fungus, disease-related growths — but this is a normal, harmless part of new wood settling in | Usually nothing needed; fades on its own over the wood's first weeks underwater |
What to do about algae
Identify and verify
-
Work out which type you're actually looking at
Use the descriptions and table above — removal methods genuinely differ between types, and confirm you're not actually looking at cyanobacteria or green water first.
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Test nitrate, and phosphate if you can
High or rapidly rising readings point toward a nutrient-management cause rather than lighting alone.
Remove what's already there
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Remove it manually
Scrape hard algae from glass, twist or pull off filamentous strands by hand or with a toothbrush, and trim badly affected leaves rather than trying to clean them in place.
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Consider algae-grazing livestock as one part of the plan
Species like Amano shrimp, bristlenose plecos and certain algae eaters can help keep regrowth down, but check a species guide for compatibility and needs first — see "what not to do" below.
Address the underlying cause
-
Review lighting duration and intensity
Reduce hours or brightness slightly and keep it on a consistent schedule; check for a direct sunlight source reaching the tank.
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Reduce feeding and reassess stocking
Less uneaten food and waste means fewer nutrients feeding algae growth.
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Keep a consistent water-change routine
Regular partial water changes export accumulated nutrients rather than letting them build up. See our guide to aquarium water changes or the Water Change Assistant.
What not to do
- Don't add an algae-eating species purely as a fix without checking its compatibility, adult size and long-term needs against your tank — see our guide to choosing tank mates first.
- Don't expect a species that eats algae to thrive long-term in a tank that's since been brought fully under control — without enough algae or supplemental food, some algae-eating species can go hungry.
- Don't reach for a chemical algaecide as a first response — a heavy die-off can deplete oxygen as it decomposes and is associated with greater disease susceptibility in aquatic animals more broadly[3], and some products carry real risk to shrimp, snails and other invertebrates.
- Don't treat every green or brown growth as the same problem — using an algae-specific product on cyanobacteria, or vice versa, is a common and ineffective mismatch.
- Don't scrub obsessively while leaving the underlying light or nutrient imbalance unaddressed — it will keep coming back.
Prevention
- Keep lighting on a consistent timer, commonly around six to eight hours a day as a starting point, and avoid direct sunlight reaching the tank.
- Feed only what's eaten within a couple of minutes, and remove obvious leftovers.
- Maintain a regular water-change routine to export accumulated nitrate and phosphate before they build up.
- In planted tanks, healthy, actively growing plants compete with algae for the same light and nutrients, so addressing plant health (light, CO2, fertilisation) often reduces algae as a side effect — see our lighting, CO2 and fertilising guides for planted-tank-specific detail.
- Rinse new decor and substrate before adding it, and clean glass on a routine schedule rather than only when algae is already established.
Frequently asked questions
Is algae in my aquarium harmful to my fish?
Usually not directly. A light dusting of algae on glass, decor or older leaves is normal and often harmless, and some fish, shrimp and snails graze on it as a food source. It becomes more of a problem when it's heavy enough to smother plants, clog equipment, or signal that light and nutrient levels are badly out of balance, which is more a tank-health and appearance issue than an immediate danger to most fish.
What's the difference between algae and cyanobacteria?
Algae are true algae — plant-like organisms that respond mainly to light and nutrients. Cyanobacteria, often called "blue-green algae" despite the name, are bacteria, not algae, and typically form a distinctive slimy sheet that peels off in one piece rather than the stringy, spotty or dusty textures true algae forms. Because they're biologically different, algae-specific treatments often don't work on cyanobacteria. See our dedicated guide to cyanobacteria in aquariums if a slimy, peeling sheet is what you're actually dealing with.
Will an algae eater fix my algae problem?
It can help, but it's not a substitute for addressing light and nutrient levels. Algae-eating species graze existing algae rather than preventing new growth, they have their own care requirements and compatibility needs, and a tank that's genuinely brought under control can leave too little algae to sustain one long-term without supplemental feeding. Treat livestock as one part of a control plan alongside lighting and nutrient management, not a stand-alone fix, and see our algae-eating fish and snails comparison for which species actually suits your tank's current and temperature before adding one.
How long should aquarium lights be on to avoid algae?
A commonly cited hobby starting point is around six to eight hours a day, but there's no single universal figure — the right duration depends on your light's intensity, how heavily planted the tank is, and how much natural light reaches it. If algae is a recurring problem, reducing duration or intensity slightly and keeping it consistent is generally more useful than chasing an exact number.
Is green water the same as an algae problem?
Green water is a type of algae problem — specifically free-floating, single-celled algae suspended in the water column rather than growing on a surface. It behaves differently to the surface-growing algae covered in this guide, including needing different removal methods, so see our guide to cloudy aquarium water for green water specifically.
How AquaPulse can help
Algae problems are usually about trends — light exposure, nitrate and phosphate creeping up over weeks — rather than a single snapshot. Recording your readings and maintenance in AquaPulse builds a history you and Water Check can refer back to, so if algae becomes a recurring issue you can compare what's changed against previous periods rather than relying on memory.
AquaPulse works from the information you enter — it doesn't take automatic readings, doesn't identify algae types from a photo, and can't tell you what's currently happening in your tank unless you've told it. Get AquaPulse on the App Store or Google Play to try it, or see the Water Check feature page for more detail.
Sources and editorial information
- Published
- 1 August 2026
- Last reviewed
- 9 September 2026
- Written and maintained by
- AquaPulse (Valeon Labs Ltd)
This guide is written and maintained by the AquaPulse team at Valeon Labs Ltd. It draws on published extension and environmental-science references rather than a single author's personal experience, and we paraphrase and link to those sources rather than reproducing their wording. Some of the source material on algae identification and nutrient dynamics comes from pond and lake-management literature rather than ornamental aquarium-specific research; we've noted this in the text wherever we've generalised from that broader evidence. We'll update this page if the sources it's based on change, or if we find something we've got wrong.
Good to know: This guide provides general informational guidance only. It is not a veterinary service and does not replace specialist or aquatic health advice for serious livestock health issues.
- [1] Penn State Extension — "Types of Aquatic Plants and Algae" Pond-management extension literature, used for general algae-type identification principles; pond-scale treatments don't translate directly to a home aquarium.
- [2] Environmental Science & Technology — "Reducing Phosphorus to Curb Lake Eutrophication is a Success" (Schindler et al., 2016) Peer-reviewed research on phosphorus as the primary limiting nutrient for freshwater algal growth, cited for the general light/nutrient mechanism; the underlying research is on lake ecosystems, not ornamental aquaria specifically.
- [3] Merck Veterinary Manual — "Environmental Diseases of Aquatic Animals in Aquatic Systems" Used for the link between organic loading, reduced dissolved oxygen and disease predisposition, relevant to a heavy algae die-off.