Algae in fish tank: how to identify and get rid of green water, hair algae, black beard algae and cyanobacteria

An algae takeover means your tank has more light or nutrients than your plants and filter can use. Identify the type, then fix the cause: cut the lights to 6–8 hours a day, feed less, and change water regularly, because every algae treatment fails if the cause stays.

Algae are simple plant-like organisms that grow wherever there is light, water and nutrients. Every aquarium has some, and a little is healthy. Algae itself rarely kills fish directly, but a takeover is a sign the tank is out of balance, and some types can smother plants or crash oxygen at night. The Wikipedia overview of freshwater aquarium algae puts it well: total elimination is unlikely, so the goal is control.

How to identify it

The type of algae you have is a clue to what's wrong:

Brown diatoms: a soft brown dust on glass, gravel and leaves that wipes off easily. Very common in new tanks (first 1–3 months).
Green water: the whole tank turns pea-soup green and you can't see through it. These are tiny floating algae. Common in new tanks, tanks in sunlight, and after nutrient spikes.
Hair or thread algae: long green strands on plants, decor and gravel. Usually too much light plus spare nutrients.
Green spot algae: hard green dots on glass and slow-growing leaves.
Black beard (black brush) algae: dark grey, black or reddish tufts, often on the edges of slow-growing leaves (Anubias, Java fern), wood and equipment. It's a red alga, despite the colour. Hard to remove. Often linked to unstable carbon dioxide (CO2) in planted tanks, and low or uneven flow.
Blue-green "algae" (cyanobacteria): slimy sheets of blue-green, dark green, brown or even red that peel off in one piece and smell musty or swampy. This isn't algae at all. It's a type of photosynthetic bacteria (Aquarium Co-op). It grows where organic waste and still water collect: dead spots, dirty gravel, low flow.

Easily confused with: a cloudy white or grey bacterial bloom (see cloudy water), tannins from wood (yellow-brown, clear, harmless), and fungus on uneaten food (white fluff).

When it's a problem

Early: the glass needs wiping more often, a light film or a few strands appear, plants stop growing well. Adjust light and feeding now.
Serious: plants coated and dying, gravel carpeted, green or murky water, fish hard to see.
Emergency: fish gasping in the morning (a sign of oxygen crashes at night, which a dense green-water bloom can cause), or a bloom that died off and left the water smelling bad.
Normal, don't worry: a little algae on glass and decor; brown diatoms in a new tank, which usually fade by themselves as the tank matures; and green spot algae, which is often harmless and a sign of a fairly balanced planted tank.

Cyanobacteria needs extra care. Some types can make toxins (US EPA), so wear gloves when you clean it out and wash your hands after.

What to do right now

Identify the type using the list above.
Test the water: ammonia, nitrite, nitrate, and phosphate if you have a kit. Algae is often a symptom of overfeeding or skipped water changes.
Cut the lights to about 6–8 hours a day, on a timer. Close curtains if sunlight hits the tank.
Remove what you can by hand: scrape glass, twirl hair algae off with an old toothbrush, siphon cyanobacteria mats, trim badly covered leaves.
Do a 25–30% water change and vacuum the gravel, especially in corners and under decor.
Feed less for the next week.
Check the flow. Clean a clogged filter sponge (in old tank water, not tap water) and point the outlet to reach dead corners.

What to do next

Fix the cause first

Light: 6–8 hours, on a timer. Avoid sunlight.
Nutrients: feed less, change water regularly, vacuum gravel. A vet-written TFH article notes that nitrate and phosphate build up between water changes and fuel algae.
Plants: fast-growing and floating plants compete with algae for nutrients and shade the water.
Flow: especially for cyanobacteria and black beard algae.
CO2 stability in planted tanks with injected CO2: keep it steady, not on and off.

Type by type

Diatoms: wipe and wait. They usually fade in weeks. Otocinclus catfish and nerite snails eat them.
Green water: water changes alone often don't clear it, because it regrows fast. A UV steriliser (an ultraviolet lamp the water flows past) clears most green water within days. Reducing light and feeding helps. Some keepers use a multi-day blackout, but watch oxygen when a bloom dies off.
Hair algae: remove by hand, cut the light, add fast plants. Amano shrimp eat many kinds.
Black beard algae: remove affected leaves; scrub decor. Spot treatment with a liquid carbon product or hydrogen peroxide on removed decor kills it (it turns pink or white, then gets eaten). Siamese algae eaters (the true Crossocheilus) eat it when young. Fix flow and CO2 or it returns.
Cyanobacteria: siphon it out, vacuum gravel, improve flow, do water changes. A 3–7 day blackout often knocks it back, but it may return and can stress plants (Aquarium Co-op). Erythromycin (an antibiotic) kills cyanobacteria and is sold in some places as an aquarium product; in others it needs a prescription. Experts disagree on whether it harms filter bacteria, so test ammonia and nitrite during use, follow the label, and treat it as a last resort.

Algaecides and cautions

Chemical algaecides can kill plants and harm shrimp and snails. When a lot of algae dies at once, it rots and uses oxygen, which can cause an ammonia spike or suffocate fish. Add an air stone and do water changes after.
Hydrogen peroxide is used by experienced keepers for spot treatments, but overdosing it burns fish gills and kills shrimp.
Copper-based algae products kill shrimp and snails.
Algae eaters help but don't fix causes. Common plecos grow to 30 cm (12 inches) or more and produce lots of waste. Choose algae eaters for your tank's size. Otocinclus, nerite snails and Amano shrimp suit most small community tanks.

Expect algae control to take 2–6 weeks. Algae returns quickly if you go back to old habits.

Products that help

Light timer: keeps lights to 6–8 hours a day without you remembering. The most useful single fix.
Algae scraper or old toothbrush: for wiping glass and twirling off hair algae.
Gravel vacuum (siphon): removes waste that feeds algae and cyanobacteria, and siphons out mats.
Test kits for nitrate and phosphate: show whether nutrients are building up between water changes.
UV steriliser: clears most green water within days.
Liquid carbon product: for spot-treating black beard algae.
Fast-growing or floating plants: compete with algae for nutrients and shade the water.
Air stone and air pump: protects oxygen when a bloom dies off or after an algaecide.
Gloves: for cleaning out cyanobacteria.

Before trouble starts, put the lights on a timer, keep a scraper and gravel vacuum handy, and add a suitable algae eater early.

How to prevent it

Put the lights on a timer for 6–8 hours.
Keep the tank out of direct sun.
Feed small amounts; remove leftovers.
Change 20–30% of the water weekly and vacuum the gravel.
Plant generously, including a fast-growing or floating plant.
Add a suitable algae eater early, matched to tank size.

Why it happened

Lights on too long, or sunlight on the tank.
Overfeeding and too few water changes, so nutrients build up.
A new tank that hasn't settled.
Few or slow-growing plants.
Poor flow and dirty gravel (especially for cyanobacteria).
Unstable CO2 in planted tanks (especially for black beard algae).

When to get help

Algae is rarely an emergency, but get help if fish are gasping in the morning, if a bloom died off and the water smells bad, or if nothing has worked after a month. A good local fish store, especially one with planted tanks, is the best help here; a vet isn't usually needed. Honest limit: some algae, like black beard, can take months to beat.

In the game

In Multiple Tank Syndrome, the Algae takeover card turns a tank green: it takes no new fish until an algae eater goes in. If an algae eater is already there, or the tank has fast-growing plants, no harm done. That's close to real life: plants and the right grazers are your best prevention.