Algae is inevitable in any aquarium — the question is whether you have a little decorative growth or a full-blown outbreak. Here’s how to identify what you’ve got and fix it permanently.
Green Spot Algae (GSA)
Green Spot Algae
Looks like: Hard, flat green dots on glass and slow-growing plant leaves
Cause: Low phosphate, too much light
Fix: GSA is actually a sign of very good water quality — it tends to appear when phosphate is too low. Slightly reduce lighting duration (to 7–8 hours/day) and if you dose fertilizers, increase phosphate slightly. Remove from glass with a razor blade scraper.
Green Water (GW)
Green Water (Algae Bloom)
Looks like: Pea-soup green water, can’t see more than an inch in
Cause: Suspended single-cell algae feeding on excess nutrients + direct sunlight
Fix: UV sterilizer is the fastest solution — clears green water in 3–5 days. Also: move tank away from direct sunlight, reduce lighting hours, do large water changes to dilute nutrients.
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Black Beard Algae / BBA (Audouinella)
Black Beard Algae (BBA)
Looks like: Dark gray or black tufts on plant edges, driftwood, and filter intakes
Cause: CO₂ fluctuation is the #1 cause; also high phosphate, low circulation
Fix: The most stubborn aquarium algae. Spot-treat affected areas with diluted hydrogen peroxide (3%) using a syringe while the tank is running — it turns pink, then dies within a week. Fix your CO₂ consistency to prevent recurrence. Siamese algae eaters will eat live BBA.
Hair Algae / Thread Algae
Hair / Thread Algae
Looks like: Long, wispy green or brown threads on plants and hardscape
Cause: Excess nutrients (especially nitrate) + inconsistent lighting schedule
Fix: Manual removal first (wrap a toothbrush in the filaments and twist). Then fix the root cause: put your lights on a timer for exactly 8 hours/day, reduce feeding, and add fast-growing plants to outcompete the algae. Amano shrimp are excellent hair algae grazers.
Brown Algae / Diatoms
Brown Algae (Diatoms)
Looks like: Soft brown film on everything — glass, substrate, plants
Cause: New tank syndrome — silicates in fresh substrate, low lighting
Fix: Almost always self-resolves within 4–8 weeks as your tank matures and silicates are depleted. Wipe off the glass, let otocinclus catfish graze on it (they love diatoms), and increase lighting slightly.
Preventing Algae: The Big Picture
| Factor | Algae-Prone | Algae-Controlled |
|---|---|---|
| Light duration | 10+ hours | 7–8 hours on a timer |
| Sunlight | Direct sun hits tank | No direct sunlight |
| Plants | Few or sparse | Dense, fast-growing |
| Feeding | Overfeeding, leftovers | Feed only what’s eaten in 2 min |
| Water changes | Infrequent | 25% weekly |