Aquarium Algae Guide – Causes, Risks & Remedies

September 05, 2025

Algae growth is one of the most common challenges in aquariums. While some algae are harmless and even natural, excessive growth can affect water quality, plant health, and the overall appearance of your tank. Identifying the algae type is the first step to solving the problem.

Brown Algae (Diatoms)

Appearance: A slimy brown coating or thin brown threads on glass, substrate, or decorations.
Cause:

Most common in newly set up aquariums with high ammonia.
Thrives in low light, non-CO₂ aquariums.
Triggered by silicates leaching from sand, cheap gravel, rocks, or tap water.
Risks: Not dangerous, but unsightly and can smother plants.
Remedy:
Wipe away manually (easy to remove).
Add Otocinclus catfish or other diatom grazers.
Improve lighting in low-tech tanks (but balance with CO₂ and fertilizers).
In new tanks, it usually disappears once the aquarium stabilizes.

Green Spot Algae (GSA – Choleochaete)

Appearance: Small, hard, round green dots on glass, rocks, and slow-growing plants like Anubias.
Cause:

Low phosphate (PO₄) levels.
CO₂ deficiency or poor circulation.
Common in well-lit, stable aquariums.
Risks: Persistent, hard to scrape, and can damage slow-growing plant leaves.
Remedy:
Manually remove from glass with an algae scraper.
Increase phosphate dosing (e.g., Seachem Flourish Phosphorus).
Relocate sensitive plants to shaded areas.
Nerite snails will help but won’t fully control it.

Green Dust Algae (GDA)

Appearance: Dusty green film on glass and hardscape, easily wiped but quickly regrows. Rarely on plants.
Cause:

Common in new aquariums.
Caused by unstable CO₂ or low nutrients.
Formed from free-floating zoospores that reattach after scraping.
Risks: Unsightly but not harmful. Frustrating because it reappears if removed too early.
Remedy:
Allow algae to grow undisturbed for 10–20 days until a thick sheet forms.
Then remove in large chunks via siphoning.
Balance CO₂ and nutrients to prevent recurrence.

Green Fuzz Algae (Oedogonium)

Appearance: Short, fine green threads (a few mm long) growing on plant leaves, especially older or damaged ones.
Cause:

Fluctuating CO₂ levels (timing mismatch with lights, unstable injection, or poor circulation).
Local nutrient deficiencies.
Risks: Can smother plants if left unchecked, indicating plants are stressed.
Remedy:
Ensure CO₂ injection starts 2–3 hours before lights on.
Trim old leaves where algae cling.
Add Amano shrimp or algae-eating fish to help control it.

Black Brush Algae (BBA – Audouinella sp.)

Appearance: Black, dark gray, or reddish tufts resembling tiny beards, often on filter outlets, rocks, and driftwood.
Cause:

Unstable or poor CO₂ distribution.
Favors high flow areas (filter pipes, powerheads).
Often linked to sudden water chemistry changes.
Risks: One of the hardest algae to remove. Plants and décor can become permanently covered.
Remedy:
Spot-treat with liquid carbon (apply directly with syringe).
Improve CO₂ stability and circulation.
Manually scrub using tools like the ADA Pro Picker.
Siamese algae eaters will graze on young tufts, but not efficiently.

Blanket Weed (Cladophora sp.)

Appearance: Coarse, moss-like green algae that spreads in thick patches. Sometimes mistaken for Marimo moss balls.
Cause:

Spread via contaminated plants or moss balls.
Thrives in areas of poor flow and excess light.
Risks: Very invasive, can overtake mosses and carpeting plants.
Remedy:
Manually remove by pulling from the root attachment point.
Trim infected moss heavily.
Improve circulation and CO₂ injection.
Prevention: only use lab-grown plants to avoid contamination.

Blue-Green Algae (BGA – Cyanobacteria)

Appearance: Slimy sheets in green, blue, reddish-purple, or black. Spreads rapidly across substrate, glass, and plants. Has a foul smell.
Cause:

Actually cyanobacteria, not algae.
Thrives in tanks with excess waste, overfeeding, or poor water changes.
Can also develop in nitrogen-rich or nitrogen-free environments.
Risks: Extremely invasive. Strips nitrates from the water, suffocates plants, and destabilizes the ecosystem.
Remedy:
Remove physically during water changes.
Improve circulation and reduce organic waste.
For severe outbreaks: a 3–4 day blackout can help.
Address nutrient balance to prevent return.

Green Water (Free-Floating Algae / Pea Soup)

Appearance: Water turns cloudy green, like pea soup, due to free-floating single-cell algae.
Cause:

Excess light combined with high nutrients, especially ammonia.
Often in new tanks or after overfeeding/fertilizer overdose.
Risks: Reduces visibility, blocks light from plants, may trigger oxygen swings.
Remedy:
UV sterilizer = most effective cure.
4–5 day blackout (with aeration) for smaller tanks.
Avoid large water changes; algae will quickly rebound.
Prevent by reducing overfeeding and stabilizing nutrients.

Hair Algae (String, Thread, Staghorn types)

Appearance: Long, hair-like strands resembling wet hair. Can form dense clumps on plants and décor.
Cause:

Excess nutrients (e.g., iron overdosing).
Too much light or imbalance between light and available nutrients.
Risks: Can suffocate plants if unchecked. Spreads quickly in high-light tanks.
Remedy:
Manually remove with toothbrush.
Balance fertilization and reduce excess iron dosing.
Reduce lighting hours.
Add algae eaters: Siamese algae eaters, Amano shrimp, mollies, flagfish.

Staghorn Algae (Compsopogon sp.)

Appearance: Branching, antler-like algae in shades of gray. Turns red in alcohol test.
Cause:

Linked to iron overdose from fertilizers.
Imbalance of macronutrients (NPK) and CO₂.
Risks: Hard to remove manually, unattractive, ignored by most algae eaters.
Remedy:
Reduce or adjust iron fertilization.
Maintain proper CO₂ (20–30 mg/L) and balanced nutrients.
Weekly 50% water changes to prevent accumulation.
Spot-treat with liquid carbon or hydrogen peroxide.

Final Tips for Algae Control

Most algae outbreaks are symptoms of imbalance (light vs. CO₂ vs. nutrients).
Prevention is easier than cure: maintain consistent routines, water changes, and balanced fertilization.
Use algae eaters as support, not as the main solution.
Always address the root cause—otherwise algae will return.

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