Aquarium algae: identify the type and fix the cause

Aquarium algae are simple photosynthetic organisms that grow on glass, rock, leaves or in the water itself when light and nutrients get ahead of what the plants can use. Every tank has a little; an outbreak is a warning sign. Identify the type, correct the cause (usually light, nutrients, flow or maintenance), then remove what is left.

Why does my aquarium have algae?

Because light and nutrients are out of balance with what your plants can use: either too much light for the nutrients on offer, or surplus nutrients from fish waste, food and rotting matter. Tropica names overfeeding as a primary cause and suggests stocking no more than about 1 cm of fish per litre, preferably less. The opposite also happens: too little fertiliser for strong light can favour algae, so cutting nutrients is not an automatic cure.

Other common triggers are long or irregular lighting, direct sunlight, water warmer than about 26 °C, weak circulation and skipped water changes. New tanks are the riskiest: Tropica treats the first 90 days as critical. A completely algae-free tank is unrealistic; a little green spot on the glass is normal even in a healthy scape.

What type of algae do I have?

Match what you see to the table. Hobby names describe appearance, not biology: black beard algae is actually a red alga, and blue-green algae is a bacterium.

What is the black fuzzy algae on my driftwood?

It is almost certainly black beard (or black brush) algae: thick tufts, usually black or grey but sometimes reddish, on wood, decor, equipment and leaf edges. There is no single agreed cause. Sources point to fluctuating or insufficient CO₂ (red algae take carbon from bicarbonate more easily than plants do), excess organic waste, light out of balance, iron or trace-element imbalance, and patches of very strong or stagnant flow; Wikipedia adds high nitrate, phosphate or iron.

Be patient: BBA takes months to establish and about 6–8 months to clear fully. Siamese algae eaters, Florida flagfish and Amano shrimp eat it (you may need many shrimp), though it is less palatable in hard water.

1. Cut off badly affected leaves.
2. For hardscape, lower the water, spray the tufts with 3 % hydrogen peroxide, wait about 5 minutes, then refill. Alternatively, spot-dose a glutaraldehyde liquid-carbon product with the filter flow off.
3. Treated tufts turn red or clear; grazers then finish them off.
4. Prevent a return with stable CO₂ that reaches full strength before the lights come on, plus regular weekly water changes.

How do you get rid of hair algae?

Remove it by hand and correct the balance of light and nutrients. Hair, thread and string algae are green filaments that look like wet hair when lifted out; staghorn algae is a branched relative. Sources disagree on the cause: Aquarium Co-Op blames excess nutrients (especially iron) with too much light, or too little fertiliser for the light, and advises more fertiliser but less iron; Wikipedia puts it mainly down to excess light.

• Twist clumps out with a toothbrush.
• Shorten the photoperiod to about 6–8 hours.
• Rebalance fertiliser, cut back iron, and change water often rather than reaching for chemicals.
• Add grazers: Amano shrimp (Tropica uses up to 2 per 5 litres while treating), Siamese algae eaters, mollies or Florida flagfish.

What is the slimy blue-green film on my substrate?

It is cyanobacteria, a photosynthetic bacterium rather than an alga. It forms a slimy blanket, usually blue-green but also brown, black or red, peels off in sheets and smells musty. It thrives in warm, stagnant, nutrient-rich water: weak circulation, dead zones behind hardscape, dirty substrate and overfeeding, with low oxygen, high phosphate and long light periods also blamed. No common algae eater touches it.

If it keeps coming back, the antibiotic erythromycin works as a chemical treatment. Follow the product label: Aquarium Co-Op uses one packet of Maracyn per 10 US gallons, repeated weekly if needed, while Fish Tank Advisor cites about 2.5 ppm for a week. Aquarium Co-Op also says it does not harm fish, plants or filter bacteria; we could not confirm that with a second source, so watch your ammonia and nitrite while treating.

1. Siphon and scrub the mats out, and vacuum the substrate.
2. Clean the filter, feed less and do more water changes.
3. Add flow with a powerhead, roughly 2–10 tank volumes an hour, to break up dead zones.
4. Try a 3-day blackout, which clears many cases.

Why is there brown algae in my new tank?

Brown algae in a new tank is almost always diatoms: a soft, flour-like brown film, sometimes short threads, on glass, substrate, stones and leaves. It is typical of the first 4–6 weeks and is linked to phosphate and silicate (Wikipedia names ammonia or silicate). It usually fades by itself as the plants grow and use up the nutrients. Meanwhile, wipe the glass, brush the leaves and let Otocinclus, snails and shrimp (Amano included) graze it.

How do you clear green aquarium water?

Green water is free-floating single-celled algae. It is not toxic in itself, but it hides the scape and can use up oxygen at night. The usual causes are too much light (especially direct sun), a fertiliser overdose or excess nutrients, or an ammonia spike from an uncycled tank or overfeeding. Water changes alone rarely clear it, because the cells multiply so fast, and no common grazer eats it.

Sources disagree on UV. Aquarium Co-Op and Buce Plant call a UV steriliser the quickest fix, about 2–3 days. Tropica discourages UV, saying it harms beneficial bacteria, and prefers water changes and fine filter material. If you want to avoid equipment, start with the blackout and a polishing pad; if that fails, UV is the fast option. Afterwards keep the light at about 6–8 hours, change at least 20 % weekly, feed less and add fast-growing or floating plants.

• Full blackout for about a week, with aeration left running (hard on plants).
• UV steriliser: clears it in about 2–3 days.
• Fine polishing pad in the filter to catch the cells.

What are the green spots and green dust on the glass?

Hard green dots that resist a sponge are green spot algae. They spread slowly on the glass and on slow-growing leaves and are common even in healthy tanks. The cause is too much light or a phosphate imbalance (Wikipedia points to low phosphate and low CO₂). Scrape the glass with a razor blade, or an old plastic card on acrylic, and let Nerite snails graze it; they leave small seed-like eggs on surfaces.

A soft green film that wipes off with a finger is green dust algae, linked to nitrogen spikes and low CO₂ and nutrients. Wikipedia simply says wipe it off. Aquasabi reportedly advises leaving it alone for about 3–4 weeks until the film loosens, because early wiping spreads spores that resettle, then lowering the water and wiping everything in one go.

How do you prevent algae in a planted tank?

Grow plants hard, keep light modest and regular, and keep a mixed crew of grazers. In Tropica's test, Amano shrimp ate about twice as much algae as Nerite snails and six times as much as cherry shrimp; Otocinclus favour diatoms and Siamese algae eaters take hair and black beard algae, so a mix covers more types.

1. Plant densely from the start, including fast growers such as Egeria and Limnophila.
2. Run about 6 hours of light for the first 2–3 weeks, then 8–10 hours on a timer, never more than 10 (Buce Plant prefers 6–8 as a general limit).
3. Change 25–50 % of the water several times a week for the first 3–4 weeks, then 20–30 % weekly with substrate vacuuming and dead-leaf removal.
4. Add snails and shrimp early; wait about 3–4 weeks before adding fish, then feed moderately.
5. If you inject CO₂, keep it steady and at full strength before the lights come on.
6. Keep the water below about 26 °C, ideally around 22 °C where your livestock allows, and the tank out of direct sun.
7. When algae appear, change one thing at a time and give it 2–3 weeks before judging.

이 가이드의 식물과 생물

• 체리새우 (네오카리디나 생이새우과) (Neocaridina davidi)
• 범블비/너라이트 스네일 / 램즈혼 달팽이 (Vittina natalensis (syn. Neritina natalensis) / Planorbarius corneus)
• 로탈라 / 레드 스템 군락 (Rotala rotundifolia / Ludwigia palustris)

관련 가이드

• Balancing light, CO₂ and fertiliser in a planted tank
• How to cycle a new aquarium (nitrogen cycle)
• Nature Aquarium 스타일: 레이아웃 규칙과 세팅 방법
• Dutch 수초 어항 스타일: 플랜트 스트리트와 레이아웃 규칙

출처

1. Aquarium algae: types and how to get rid of them — Aquarium Co-Op
2. Black beard algae — Aquarium Co-Op
3. Hair algae — Aquarium Co-Op
4. Blue-green algae — Aquarium Co-Op
5. Brown algae (diatoms) — Aquarium Co-Op
6. How to get rid of green water — Aquarium Co-Op
7. Types of algae — Tropica Aquarium Plants
8. Preventing algae — Tropica Aquarium Plants
9. Starting a new aquarium — Tropica Aquarium Plants
10. Test of algae eaters — Tropica Aquarium Plants
11. Freshwater aquarium algae — Wikipedia
12. Algae in aquariums: causes, common types, and effective solutions — Buce Plant
13. How to get rid of green water in aquariums — Buce Plant
14. Cyanobacteria in aquariums: causes, prevention and treatment — Fish Tank Advisor
15. How to get rid of Black Beard Algae (BBA)? — Aquascape Guide
16. Green dust algae (GDA) — Aquascaping Wiki — Aquasabi
17. Black beard algae (BBA) — Aquascaping Wiki — Aquasabi

이 출처들을 바탕으로 작성; 최종 업데이트 2026-09-23.

리빙 월드 둘러보기 · 수초·생물 관리 가이드 · 아쿠아스케이핑 가이드 · 591개 실제 수초 어항 분석 데이터 · 3D 에디터 열기