Algae is the most frustrating part of the aquarium hobby, but every type of algae is telling you something specific about your tank's conditions. Identify the type correctly and you can fix the root cause rather than just scrubbing the glass every week. Here's every common algae type, what causes it, and how to eliminate it.
Why Algae Grows in the First Place
Algae grows when light and nutrients are present without being fully consumed by plants. The solution is almost never scrub more; it's restoring balance between your light, CO2, and nutrients. Every algae type is a symptom of a specific imbalance.
Green Spot Algae (GSA)
Looks like: small, hard green dots on glass and slow-growing plant leaves.
Cause: low phosphate, or excessive light hitting a specific area.
Fix: increase phosphate slightly, or reduce lighting duration. Physically remove from glass with a razor blade scraper.
Green Dust Algae (GDA)
Looks like: a light green film or dust covering the glass, wipes off easily.
Cause: common in new tanks; often disappears on its own as the tank matures.
Fix: leave it for 3–4 weeks without wiping; it will often crash and disappear. Improve plant coverage. Introduce nerite snails.
Hair Algae / Thread Algae
Looks like: long, stringy green threads growing from plants, substrate, and décor.
Cause: high light, low CO2, or nutrient imbalance.
Fix: increase CO2 injection, reduce light intensity or photoperiod, manually remove as much as possible. Introduce Amano shrimp; they eat thread algae aggressively.
Black Beard Algae (BBA)
Looks like: dark grey or black tufts, like a tiny beard growing on plant edges, décor, and filter intakes.
Cause: CO2 fluctuation, typically occurring when CO2 drops during the lights-on period.
Fix: stabilise CO2. Spot-treat affected areas with liquid carbon using a syringe with the pump off. BBA will turn red/pink and die within 3–5 days, then shrimp and fish will eat it.
Cyanobacteria (Blue-Green Algae)
Looks like: a slimy, blue-green coating with a distinctive musty smell.
Cause: despite the name, this is bacteria, not algae. Common causes: low nitrate, poor circulation, too much light.
Fix: manually remove as much as possible, increase water flow, do a 3-day blackout. Address the root cause or it will return.
Brown Algae (Diatoms)
Looks like: a brown, dusty coating on glass and leaves, wipes off easily.
Cause: extremely common in new tanks and low light conditions.
Fix: almost always disappears on its own within 4–6 weeks as the tank matures. Otocinclus catfish and nerite snails eat diatoms.
The Best Algae-Eating Animals
- Amano shrimp: the most effective algae eaters available, particularly for thread and hair algae
- Nerite snails: excellent for GSA, diatoms, and green dust algae. Cannot breed in freshwater
- Otocinclus catfish: ideal for diatoms and soft green algae
- Siamese algae eater: one of the few species that eats BBA
- Bristlenose pleco: good for green algae on glass and wood
Prevention: The Long-Term Solution
- Run lights no more than 6–8 hours per day on a timer
- Keep plants dense; healthy plants outcompete algae for nutrients
- Maintain stable CO2 if injecting
- Do regular water changes to control nutrient levels
- Never place the tank in direct sunlight
Find algae scrapers, UV sterilisers, CO2 systems, and liquid carbon in our Aquatics range.
