Algae control: types and how to fight them
Green, black, brown, beard — each algae type has its own trigger and treatment. Identify by sight, fix the cause.
Algae aren't a disease — they're a signal of imbalance. 'Anti-algae products' just mask the symptom. This guide covers the 5 main types: how to identify them and what to fix.
1. Green dust algae (GDA)
Symptom: dust of tiny green dots on the glass, returns within 3–5 days after cleaning. Cause: excess light, unstable CO₂. Fix: don't clean for 3 weeks — the spores mature and slough off as a single sheet that wipes off in one stroke. Then cut the photoperiod to 6 hours.
2. Brown algae (diatoms)
Symptom: brown 'dust' on glass, leaves, and substrate in a new tank (2–6 weeks in). Cause: excess silicates in fresh water, immature biofilter. Fix: wait. Resolves on its own in 4–8 weeks. Speed it up with nerite snails and otocinclus.
3. Black beard algae (BBA)
Symptom: dark grey-black 'beards' on leaf edges, equipment, and driftwood. Cause: unstable or insufficient CO₂, poor flow. Fix: stabilise CO₂ at 25–30 mg/L, improve circulation. Spot treatment: glutaraldehyde (Excel) or 3% hydrogen peroxide directly on the affected area (filter off, 5 minutes).
4. Hair / thread algae
Symptom: thin green threads on plants and substrate, sometimes forming dense mats. Cause: too much light + nitrogen/phosphate deficit (unbalanced fertilisation). Fix: pull it out manually (a toothbrush works well), balance macros (NO₃ 10–20 mg/L, PO₄ 0.5–1 mg/L). Amano shrimp are the best clean-up crew.
5. Green water (phytoplankton bloom)
Symptom: water turns cloudy green, fish are barely visible. Cause: direct sunlight, excess nitrates, overcrowding, low pH plus organics. Fix: run a UV steriliser for a week (kills suspended plankton) + 3-day blackout (cover the tank with a dark cloth).
Overall strategy
1. Identify the type from a photo and description. 2. Fix the cause, not the symptom. 3. Wait 2–4 weeks — algae retreat slowly. 4. In parallel, recruit clean-up crew: amano shrimp, otocinclus, nerite snails.
Algae are always faster than plants. If they're growing, your plants are short of something algae aren't. Treat the plants, not the algae.
FAQ
- Do 'anti-algae' products work?
- Most are toxic to plants and invertebrates above the recommended dose. They knock the symptom down for 2–4 weeks, but algae return unless the cause is fixed. Only truly effective for BBA (Excel/glutaraldehyde) and green water (UV).
- How many Amano shrimp for a 100 L tank?
- 6–8 individuals. Fewer can't keep up with hair algae. More and they start competing for residual food themselves.
Scientific board — ichthyologists and veterinarians
Ichthyologists and veterinarians with university degrees · Reference FishBase, Seriously Fish and peer-reviewed literature · Sign every reviewed article with their credentials shown
Goldie editorial team
Practising aquarists with a combined 30+ years of experience · Biologists and editors, fact-checking against FishBase and Seriously Fish · Every piece is reviewed by a qualified ichthyologist before publication
Sources
- 2hraquarist — Algae types · 2HR Aquarist · 2026-05-22
- Aquarium Co-Op — Algae guide · Aquarium Co-Op · 2026-05-22