The Complete Guide to Eliminating Aquarium Algae: Every Species, Identified and Defeated
Algae is the most persistent challenge in the planted aquarium hobby. Whether you are a seasoned aquascaper or setting up your first Nature Aquarium, at some point you will face an unwanted bloom. The key to winning the battle is understanding your enemy — because not all algae are the same, and what works against one species may do nothing against another.
This guide covers every major algae species found in freshwater planted aquariums, one by one: how to identify it, what causes it, and precisely how to eliminate it. Bookmark it — you will come back to it.
1. Green Spot Algae (GSA) — Coleochaete orbicularis
What it looks like
Small, hard, perfectly circular spots of bright green algae that adhere firmly to the aquarium glass, slow-growing plant leaves (such as Anubias and Bucephalandra), and hardscape. They are extremely difficult to remove with a soft cloth and require a razor blade or stainless steel scraper. The spots are typically 1–3 mm in diameter and have a slightly raised, lens-like profile.
Why it appears
- Phosphate deficiency (PO₄ below 0.1 ppm) — this is the primary trigger
- Excessive or prolonged lighting periods
- Slow plant growth leaving nutrients unbalanced
- Hard water with high calcium levels
How to eliminate it
- Raise phosphate to 0.5–1.0 ppm using monopotassium phosphate (KH₂PO₄)
- Reduce your photoperiod to 6–8 hours; consider a midday siesta
- Scrape the glass weekly with a stainless steel blade
- Introduce Nerite snails (Neritina natalensis) — one snail per 10 litres
Long-term prevention
Maintain phosphate above 0.2 ppm at all times. GSA is almost always a phosphate deficiency signal.
2. Green Hair Algae / Thread Algae — Spirogyra spp. & Pithophora spp.
What it looks like
Long, soft, bright green filaments that grow from plant leaves, substrate, and hardscape. Spirogyra is silky to the touch with a characteristic spiral chloroplast; Pithophora is coarser and more rope-like.
Why it appears
- Excess ammonia — common in new tanks or after adding new substrate
- Insufficient or unstable CO₂ injection
- High light with inadequate plant mass
- Nutrient imbalance
How to eliminate it
- Manually remove with a toothbrush or wooden chopstick
- Stabilise CO₂ to 25–30 ppm
- Introduce Amano shrimp (Caridina multidentata) — minimum 10 per 60 litres
- Dose liquid carbon (Easy Carbo / Excel) at double rate for 2 weeks
- Perform 50% water changes twice weekly
3. Black Beard Algae (BBA) — Audouinella spp.
What it looks like
Dense, dark grey to black tufts resembling a beard, growing on plant leaf edges, driftwood, and filter intakes. Despite its colour, BBA is a red algae — it turns bright pink when treated with hydrogen peroxide or glutaraldehyde.
Why it appears
- Fluctuating or insufficient CO₂ — the primary cause
- Poor water circulation creating CO₂-depleted dead zones
- Organic waste accumulation
How to eliminate it
- Stabilise CO₂ — use a solenoid on a timer
- Apply glutaraldehyde (Easy Carbo / Excel) directly with a syringe outside the water
- Introduce Siamese Algae Eaters (Crossocheilus oblongus)
- Remove heavily infested leaves entirely
- Improve flow with a powerhead
4. Cyanobacteria (Blue-Green Algae — BGA)
What it looks like
A slimy, sheet-like film in shades of blue-green or dark teal with a distinctly unpleasant, musty odour. It spreads rapidly and can be peeled away in sheets. Technically photosynthetic bacteria, not true algae.
Why it appears
- Very low nitrate levels (NO₃ below 5 ppm)
- Poor water circulation and stagnant areas
- Organic waste build-up in the substrate
- New tank syndrome
How to eliminate it
- Siphon out manually during water changes
- Raise nitrates to 10–25 ppm using KNO₃
- Improve circulation — no dead spots
- Blackout method: 3 days complete darkness + 50% water change before and after
- Erythromycin as a last resort only
5. Brown Algae / Diatoms — Bacillariophyta
What it looks like
A soft, powdery, light to dark brown coating on glass, substrate, and leaves. Wipes off very easily — this is its key identifying feature. Extremely common in new aquariums during the first 4–8 weeks.
Why it appears
- New tank syndrome
- High silicate (SiO₂) levels in tap water
- Insufficient lighting intensity
How to eliminate it
- Be patient — diatoms disappear on their own within 4–8 weeks
- Introduce Otocinclus catfish — specialist diatom grazers
- Switch to RO water if silicates are persistently high
- Increase lighting intensity
6. Green Water (Algae Bloom) — Chlorella spp.
What it looks like
The entire water column turns opaque green, resembling pea soup. Caused by a massive bloom of free-floating, single-celled green algae. Cannot be scraped — must be addressed at the water level.
Why it appears
- Direct sunlight hitting the tank
- Excess nutrients with insufficient plant competition
- Inadequate filtration
How to eliminate it
- UV steriliser — clears green water within 24–72 hours
- Blackout method: 4 days complete darkness + 50% water change
- Eliminate all sources of direct sunlight
- Add fast-growing stem plants
7. Staghorn Algae — Compsopogon spp.
What it looks like
Grey to whitish, stiff filaments branching in a distinctive Y-shape resembling miniature stag's antlers. Lighter in colour than BBA and clearly structured. Individual filaments reach 1–3 cm in length.
Why it appears
- Low or fluctuating CO₂ — identical to BBA
- Poor flow in specific tank areas
- High ammonia spikes
How to eliminate it
- Stabilise and increase CO₂
- Apply glutaraldehyde directly — staghorn turns red/pink within 48 hours
- Improve circulation
- Amano shrimp will consume weakened staghorn
8. Cladophora — Cladophora spp.
What it looks like
Dense, rigid, dark green masses with a rough, scratchy texture. Stiff and branching — very different from soft hair algae. Frequently introduced via contaminated plants or equipment.
⚠️ Do not confuse with Aegagropila linnaei (Marimo moss balls) — Marimo is spherical and velvety; Cladophora is irregular and scratchy.
How to eliminate it
- Remove manually — wear gloves
- Treat with 3% hydrogen peroxide (H₂O₂) outside the tank for 3–5 minutes
- Dose liquid carbon at elevated rates for 2–4 weeks
- Bleach dip new plants (1:20, 2 minutes) before introduction
9. Fuzz Algae — Rhizoclonium spp.
What it looks like
Short (2–5 mm), fuzzy, light green filaments growing uniformly over plant leaves and hardscape, giving a distinctly furry or velvety appearance. Soft to the touch and comes away easily when rubbed.
Why it appears
- Moderate to high light with insufficient CO₂
- New tank or recently disturbed substrate
- Low plant competition
How to eliminate it
- Increase and stabilise CO₂
- Introduce Amano shrimp and Otocinclus
- Reduce photoperiod to 6 hours temporarily
- Dose liquid carbon for 2–3 weeks
The Golden Rules of Algae Prevention
- The Limiting Factor Rule: light, CO₂, and nutrients must always be in proportion
- Plant densely from day one — plants are your primary defence
- Maintain a consistent routine: weekly 30–50% water changes, stable CO₂, balanced fertilisation
- Quarantine everything — plants, hardscape, and equipment can all introduce algae
- Test your water regularly — NO₃, PO₄, CO₂, and silicates weekly
- Address root causes, not symptoms — scraping without fixing the imbalance is temporary
- Build the right clean-up crew: Amano shrimp, Otocinclus, Nerite snails, Siamese Algae Eaters
- Be patient — new tanks go through algae phases. Consistency always wins
Quick Reference: Algae Identification & Treatment
| Algae | Appearance | Primary Cause | Best Treatment |
|---|---|---|---|
| Green Spot (GSA) | Hard green circles on glass | Low phosphate | Raise PO₄, Nerite snails |
| Green Hair | Long soft green filaments | Unstable CO₂, excess NH₄ | Amano shrimp, liquid carbon |
| Black Beard (BBA) | Black/grey tufts on leaf edges | Fluctuating CO₂ | Stabilise CO₂, glutaraldehyde |
| Cyanobacteria | Blue-green slimy film, bad odour | Low nitrates, poor flow | Blackout, raise NO₃ |
| Brown/Diatoms | Powdery brown coating | New tank, high silicates | Patience, Otocinclus, RO water |
| Green Water | Opaque green water | Direct sunlight, excess nutrients | UV steriliser, blackout |
| Staghorn | Grey-white branching filaments | Fluctuating CO₂ | Stabilise CO₂, glutaraldehyde |
| Cladophora | Rigid dark green scratchy clumps | Contamination | H₂O₂ treatment, bleach dip |
| Fuzz Algae | Short fuzzy uniform coating | Low CO₂, new tank | Amano shrimp, Otocinclus |
Struggling with a persistent algae problem? Leave a comment below with your tank parameters (light, CO₂, NO₃, PO₄, photoperiod) and a photo — our team will help you identify and tackle it.