Brown dust on the sand of a tank six weeks old. Hard green dots on the glass that a cloth will not shift. A fine green haze back within a day of wiping it. Black tufts on the driftwood and along the edges of the anubias leaves. Long green strands wound round a filter outlet. A slimy sheet in the back corner that peels away whole and smells like a pond.
Six symptoms, six different causes, and most guides on aquarium algae types and fixes answer all of them with the same list of products. That is the wrong order.
Algae is not a disease that infects a tank. It grows when the light energy going in exceeds what your plants can convert, when nutrients sit unused, or when a dead spot lets organic waste accumulate. Spores are already in your tank and always will be. They bloom when conditions invite them.
So the scrubbing is the easy half. The half that decides whether you do this again next month is working out what the algae is telling you, and the types are genuinely diagnostic. Brown dust in a six-week-old tank means something completely different to black tufts on a filter outlet.
The short answer
Identify the algae first, because each type points at a different cause. Brown dusty film in a new tank is diatoms and usually passes on its own. Hard green dots on glass are green spot algae, typically strong light plus low phosphate. A fine green haze that returns within a day is green dust algae. Dark tufts on hardscape and leaf edges are black beard algae, a red algae linked to unstable CO2 and poor flow. Green strands are hair algae, from light and nutrients out of step with plant growth. Slimy sheets that peel away whole and smell swampy are cyanobacteria, a bacterium rather than an algae at all.
Then pull levers in this order: shorten the photoperiod to six or seven hours, reduce light intensity, correct the nutrient imbalance, fix flow and dead spots, add more fast-growing plants. Clean-up crews help. They do not fix causes.
Algae is a symptom, and the symptom is usually about light
Take one thing from this: light is the throttle, and almost every persistent algae problem I have seen has too much of it, for too long, for the plant mass available.
Darkness does not fix everything. But light is where you start, because it is the variable you control most precisely and the one hobbyists most often get wrong, usually by running a bright LED for ten hours because that is when they are home.
The balance has three parts. Light provides energy. Nutrients (nitrogen, phosphorus, potassium, iron, traces) provide the building blocks. Carbon, from injected CO2 or the small amount dissolved in any tank, is the limiting material in most planted setups. When all three line up and there is enough healthy plant mass, plants outcompete algae. When one falls out of step, algae gets the surplus, because algae is far less fussy than a stem plant.
Here is where the hobby genuinely disagrees, and you should know it before someone tells you otherwise with total confidence. One camp holds that algae is caused by excess nutrients, so you dose lean and keep nitrate low. Another, associated with the Estimative Index approach, deliberately keeps nitrate at 10 to 20 ppm and phosphate at 0.5 to 2 ppm, on the argument that starving plants causes algae faster than feeding them does. Both camps run beautiful tanks. What they share is high plant mass, consistent CO2 for their light level, and good flow. Neither is settled science.
The identification table
Work from appearance and texture. Texture separates types that look identical in photos.
| Type | Appearance | Texture | Usual trigger | First fix |
|---|---|---|---|---|
| Diatoms (brown algae) | Dusty brown coating on glass, sand and leaves | Wipes off with a finger, clouds the water | New tank under 3 months, silicates, low competition | Wait it out, wipe glass, light substrate siphon |
| Green spot algae | Hard green dots, 1-2 mm, on glass and old anubias leaves | Will not wipe off, needs a blade | High light on slow-growing surfaces, low phosphate | Dose phosphate to 0.5-1 ppm, scrape glass, add nerites |
| Green dust algae | Even green haze on the front glass, back within a day | Smears when wiped | Strong light in a tank still finding balance | Leave it 3 weeks, then wipe and do a large change |
| Black beard algae | Dark grey to black tufts 2-15 mm on wood, leaf edges, outlets | Tough, wiry, does not squash | Fluctuating CO2, high organics, uneven flow | Stabilise CO2, cut light, spot-dose peroxide |
| Hair and thread algae | Green strands from 1 cm to 20 cm | Soft and slippery, or wiry if Cladophora | Excess light with nutrients out of balance | Twirl out by hand, cut photoperiod, add amanos |
| Cyanobacteria | Blue-green or reddish sheets over substrate and glass | Slimy, peels off whole, smells swampy | Low flow, high organics, sometimes very low nitrate | Siphon out, 3-4 day blackout, fix flow |
| Green water | The water itself is green, objects hazy at distance | Suspended, attached to nothing | Light and nutrient spike, often after a substrate disturbance | Blackout or UV, not water changes alone |
Green water gets its own treatment in our post on cloudy aquarium water causes, because people mix it up with bacterial blooms constantly.
Brown diatoms in a new tank
If your tank is under three months old and everything is coated in brown dust, this is diatoms and it is close to a rite of passage. It appears on glass, on sand, on the leaves of anything slow-growing, and it wipes away with a finger leaving a brown cloud in the water.
The usual explanation is silicate: silica leaches from new substrate, sand and tap water, diatoms use it to build their frustules, and once that supply is depleted they fade. That story is broadly right for the new-tank phase. I would treat it sceptically as an explanation for diatoms in a two-year-old tank, where low light and immature biofilm are likelier.
Do very little. Wipe the front glass, siphon the top of the substrate during your weekly change, and wait. Most tanks clear within two to eight weeks as the filter matures and plants start pulling their weight.
Otocinclus (Otocinclus vittatus) and nerite snails (Neritina natalensis) both graze diatoms enthusiastically. Do not rush to buy otos for a tank under a couple of months old, though: they need established biofilm, and adding them in week four usually ends with dead otocinclus and the diatoms clearing on schedule anyway.
Green spot algae and green dust algae
These two get confused because both are green and both live on the front glass. The texture tells them apart immediately.
Green spot algae
Hard, raised dots roughly 1 to 2 mm across, firmly attached, refusing to wipe off. They appear on glass and on the oldest leaves of slow growers, especially anubias, a leaf that sits under bright light for months without producing new tissue.
The common association is strong light plus phosphate that has dropped near zero. In a dosed planted tank, raising phosphate to roughly 0.5 to 1 ppm usually stops new spots forming within a fortnight, though existing spots stay until you remove them. On glass, a razor blade (or a plastic blade on acrylic) takes them off in one pass. On anubias, badly spotted old leaves are best trimmed at the rhizome.
Green dust algae
A fine, even green haze on the glass that smears rather than lifts, and returns a day or two after cleaning. Wiping it daily feels productive and, going by how most tanks respond, makes the cycle repeat.
The widely used approach is counterintuitive: leave the glass alone for around three weeks. The theory is that the algae completes its life cycle, releases zoospores and sloughs off, at which point you wipe the glass, do a large water change and vacuum out the fallen material. I have seen it work often enough to recommend it, though the mechanism people quote is hobby lore rather than anything I can point you to.
Black beard algae, the one people actually search for
Black beard algae, often shortened to BBA, is not a green algae at all. It belongs to the red algae, genus Audouinella, which is why it can look almost purple in bright light and why it turns pink as it dies.
It grows as dense tufts, 2 to 15 mm long, colonising where flow is strongest: filter outlets, the leading edges of hardscape, the tips of older leaves, the intake strainer. That distribution is the clue. Black beard is associated with fluctuating CO2 and a build-up of organics, and it settles where water movement delivers most of both.
If you inject CO2, the useful change is stability rather than quantity. A drop checker that is green at midday and blue in the morning means your plants get a moving target, and moving targets favour algae. Start the gas an hour or two before lights on, and stop it about an hour before lights off. If you do not inject CO2, run a lower light level, because a bright light over a tank with no carbon supplement is the classic black beard setup.
For what is already there, three approaches, in order of how much I trust them.
Take the surface out of the tank. Badly affected leaves get trimmed. A rock or piece of wood can be lifted out, dosed with 3% hydrogen peroxide, left two or three minutes, rinsed in dechlorinated water and replaced. This is safest, because none of the peroxide reaches your fish.
Spot-dose in the tank. Turn off filters and pumps, draw 3% peroxide into a syringe, and squirt it directly onto tufts you can reach. Start low: around 0.5 ml per US gallon (0.13 ml per litre) of total tank volume, and do not go past roughly 1.5 ml per gallon (0.4 ml per litre) in a single day even in a densely planted tank. Wait 15 minutes, then restart flow. Peroxide is an oxidiser, and the damage it does to gills at high concentration is the same chemistry that kills the algae, so treat the dose as a real limit rather than a suggestion. Shrimp, mosses and vallisneria are the first casualties if you overdo it. If any fish start gasping at the surface, do a large water change immediately.
Glutaraldehyde-based liquid carbon products work similarly as spot treatments. Whether they supply usable carbon at all is contested, and I treat them as algaecides with an optimistic label. They kill vallisneria and some mosses at normal doses.
Dying black beard goes red, then white, over three to seven days. If it is still black after a week, the dose did not reach it.
Hair and thread algae
Green strands, anywhere from a centimetre to twenty, draped over plants and hardscape. Feel it before deciding what to do. Soft, slippery strands are usually Oedogonium or Spirogyra and respond well to manual removal plus a nutrient and light correction. Rough, wiry strands that feel like wet wool are probably Cladophora, which ignores nearly everything.
Manual removal works better than it sounds. A toothbrush twirled into the strands lifts a surprising volume in one go. Do it at the start of a water change so the debris leaves with the old water.
Then look at the imbalance. Hair algae turns up when light is generous but something else has run short, so plants stall and the surplus goes to algae. In a dosed tank, check nitrate has not hit zero and that iron is being replaced. In a low-tech tank, cut the photoperiod first.
Amano shrimp are genuinely good at soft hair algae, and ten in a 20 gallon (75 litre) tank make visible progress in a fortnight. Florida flagfish (Jordanella floridae) and rosy barbs (Pethia conchonius) also graze it, though flagfish nip plants and slow tank mates, and rosy barbs want cooler water at 64-72°F (18-22°C).
Cyanobacteria is not an algae
This matters practically, not just pedantically. What the hobby calls blue-green algae is cyanobacteria: a photosynthetic bacterium. It is why no algae eater touches it, why it grows in sheets rather than filaments, and why treatments that work on true algae often do nothing.
Three signs identify it. It forms a slimy mat that peels off the substrate whole rather than in strands. It is blue-green, sometimes with reddish patches. And it smells: earthy, musty, swampy, unmistakable once met.
The triggers, in rough order of how often they get blamed: poor flow, meaning dead spots behind hardscape and in corners; organic waste built up in the substrate, usually from overfeeding; and, contested but widely reported, nitrate at or near zero, since cyanobacteria can fix atmospheric nitrogen while your plants cannot.
The treatment sequence that works most reliably:
- Siphon out every piece you can reach. It lifts in sheets, so get under an edge and peel.
- Do a 40% water change (in a 29 gallon or 110 litre tank, about 12 gallons or 44 litres) and vacuum the affected substrate.
- Cut feeding back sharply for a fortnight. Most tanks with cyanobacteria are being overfed.
- Blackout the tank for three to four days: lights off, the whole thing wrapped in bin bags or a blanket so no room light gets in. Keep the filter running and add an airstone. Do not feed for the first two days, then lightly.
- Uncover, do another 40% change, and siphon out the dead material, which will be dark and loose.
- Fix flow before it returns: a small circulation pump aimed at the dead corner, or a repositioned filter outlet.
- Test nitrate. If it reads zero in a planted tank, dose potassium nitrate to 5 to 10 ppm.
Set the blackout with a countdown timer so you are not guessing at the end of day three.
The erythromycin question
Erythromycin, an antibiotic, kills cyanobacteria effectively. It is also the option I would reach for last, and only after a proper blackout plus flow correction has already failed.
The trade-off is your biological filter. Erythromycin targets bacteria, and the nitrifying bacteria doing the ammonia and nitrite work in your filter are bacteria. Reports from keepers range from no measurable effect to a full cycle crash, and nobody can tell you in advance which you will get. If you use it, test ammonia and nitrite daily for a week afterwards and be ready to change water. There is also the broader argument about casual antibiotic use driving resistance.
Availability differs by country. In the UK and much of the EU, antibiotics for ornamental fish are veterinary medicines rather than shelf products, and in the United States the availability of over-the-counter fish antibiotics has changed over time. Check what applies where you live and speak to an aquatic vet if you are unsure.
The levers, in the order they actually matter
Change one thing at a time and give it two weeks. Four changes at once teaches you nothing.
Photoperiod comes first because it is free and reversible. Eight hours is common, ten is too many, and dropping to six or seven for a month is the single most effective intervention available to you.
Light intensity is second. Raising a fixture by 10 cm, or dimming it, cuts the energy reaching the substrate substantially. Many modern LED units are far brighter than the tanks under them need.
Nutrients third. In a planted tank, the aim is that nothing runs out: nitrate present rather than zero, phosphate present rather than zero, iron and traces replaced weekly. In an unplanted tank the aim is the opposite, keeping nitrate low through water changes, which is where the aquarium water change schedule matters. Two tanks, two opposite instructions, and this is why generic algae advice fails so often.
Flow fourth. Every dead spot is a candidate site. Aim for gentle movement reaching all four corners.
Plants fifth, and underrated. A mass of fast-growing stems (hornwort, Limnophila sessiliflora, water sprite) soaks up surplus nutrients quickly, and you can pull them out later.
Stocking and feeding sit underneath all of it. A tank stocked beyond what its plants and water changes can absorb will keep producing algae no matter what you do to the light. The Aquarium Stocking Calculator gives you a read on bioload against tank volume and filtration in a couple of minutes, running entirely in your browser with nothing uploaded and no account needed.
The clean-up crew, honestly assessed
| Animal | Eats well | Ignores | Honest note |
|---|---|---|---|
| Nerite snail (Neritina natalensis) | Green spot, diatoms, green dust | Hair algae, BBA, cyano | The most useful glass cleaner in the hobby |
| Amano shrimp (Caridina multidentata) | Soft hair algae, biofilm | BBA, green spot, cyano | Needs a group of 8+ to show results |
| Otocinclus (Otocinclus vittatus) | Diatoms, soft green film | Anything tough | Fragile on import, mature tanks only |
| Siamese algae eater (Crossocheilus oblongus) | BBA when young, hair algae | Green spot | Reaches 6 in / 15 cm, often mis-sold as flying fox |
| Bristlenose pleco (Ancistrus sp.) | Soft green film, diatoms | BBA, cyano | Adds a big waste load, which can worsen things |
| Ramshorn and bladder snails | Dying plant matter, biofilm | Most established algae | They follow excess food, they do not cause it |
The Siamese algae eater needs a caution. Crossocheilus oblongus genuinely eats black beard, mainly while young and hungry; adults switch to prepared food. The similar-looking flying fox (Epalzeorhynchos kalopterus) does not eat BBA and gets territorial. Check for the black stripe running through the tail fin rather than stopping at its base.
A worked reset
Say Rosa has a 29 gallon (110 litre) planted tank, 30 by 12 by 18 inches, at 78°F (25.5°C) with no CO2 injection. Her LED runs 9 hours a day. Anubias and java fern on wood, some cryptocoryne, a few stems. Black beard algae is spreading across the wood and the filter outlet, and there is green spot on the front glass.
Week one, she changes two things and nothing else: photoperiod from 9 hours to 6.5, and the fixture raised 10 cm on its legs. She lifts the two smallest pieces of wood out, dips the affected areas in 3% peroxide for three minutes, rinses and replaces them.
She also spot-doses the outlet, which she cannot remove. At 0.5 ml per gallon, 29 gallons means 14.5 ml of 3% peroxide, drawn into a 20 ml syringe and applied with the filter off, restarted 15 minutes later.
Week two, the treated black beard has gone red and is whitening. She adds hornwort and water sprite, scrapes the green spot off the glass with a blade, and starts dosing phosphate to 0.5 ppm, because green spot under this much light usually means phosphate is bottoming out.
Week four, the wood is clean apart from a few tufts near the outlet, which she treats once more. Green spot has not returned. She takes the photoperiod back to 7.5 hours and watches for a fortnight.
The part worth copying is not the numbers. It is that she changed light first, treated second, and gave each change two weeks before judging it.
What goes wrong
Doing everything at once. If you cut light, change dosing, add shrimp, deep-clean the substrate and treat with peroxide in one weekend, and it improves, you have learned nothing about your tank.
Deep-cleaning the substrate in response to cyanobacteria. Stirring a mature substrate releases a pulse of organics and makes things worse. Siphon the surface and the visible mat; leave deeper layers alone.
Buying algae eaters as the primary fix. Every animal in that table adds waste to a tank that already has a nutrient surplus. Useful staff, not a solution.
Assuming a UV steriliser handles everything. UV kills what passes through it, meaning free-floating green water. Attached algae never goes near the unit.
Ignoring the supply line. If nitrate climbs steadily between changes, algae has a steady feed, and the aquarium bioload calculator tells you whether the tank carries more fish than it should.
And one that catches careful people: dosing peroxide by eye. Measure it. If the instructions are in millilitres per gallon and your tank is quoted in litres, a unit converter removes the chance of a factor-of-four error, which here is the difference between clearing algae and burning gills. None of this transfers to marine tanks, where the Reef Dosing Calculator is the more relevant tool.
Frequently asked questions
How do I get rid of black beard algae?
Cut light duration to six or seven hours, stabilise CO2 if you inject it, and improve flow so no dead spots remain. Then treat what is there: remove the worst leaves, and spot-dose 3% hydrogen peroxide or liquid carbon onto tufts with the pumps off. Dying black beard turns red then white over three to seven days.
Why is my new tank covered in brown algae?
That is almost certainly diatoms, a normal phase in tanks under about three months old. They coat glass, sand and leaves in a dusty brown film that wipes away easily. They fade on their own as the tank matures and silicate is used up. Wipe the glass, siphon the substrate lightly, and add otocinclus or nerite snails once the tank is established.
Is blue-green algae in an aquarium actually algae?
No. Blue-green algae is cyanobacteria, a photosynthetic bacterium rather than a true algae, which is why algae eaters ignore it. It forms slimy sheets that peel off whole and smells earthy or swampy. Treatment is manual removal, a three to four day blackout, and fixing the low flow or high organics that let it start.
What causes green spot algae on the glass?
Hard green dots on glass and on slow-growing leaves such as anubias are usually strong light plus low phosphate. In a planted tank, dosing phosphate to roughly 0.5 to 1 ppm often stops new spots forming. Scrape existing spots off glass with a blade, never on acrylic, and let nerite snails handle the rest.
What eats hair algae in a freshwater tank?
Amano shrimp, Caridina multidentata, are the most reliable, and ten of them visibly reduce soft hair algae in a 20 gallon or 75 litre tank. Florida flagfish and rosy barbs also graze it, though both can nip plants and fins. No animal touches Cladophora or cyanobacteria, and none fix the underlying cause.
How do I dose hydrogen peroxide without hurting the fish?
Use 3% household peroxide, turn off filters and pumps, and syringe it directly onto the algae. Start low, around 0.5 ml per US gallon or 0.13 ml per litre of tank volume, and do not exceed roughly 1.5 ml per gallon in one day. Restart flow after 15 minutes. Shrimp, mosses and vallisneria are sensitive, so stop if any fish gasp.
Where to start this weekend
Name what you have, using texture rather than colour. Then change the light and only the light: photoperiod down to six or seven hours and, if the fixture is adjustable, raise or dim it.
Give it a fortnight before touching anything else. Do your normal water changes, feed a little less than feels right, and pull out by hand whatever you can reach.
Most tanks that have been fighting algae for months are running a light too strong for too long over too few plants, and correcting that quietly resolves things no product would have. If it does not, you will at least be changing the next variable from a known baseline.