Algae guide

Why identify algae

Algae, like aquatic plants, are photosynthetic organisms. A small amount of algae is part of a healthy ecosystem—they absorb excess nutrients, provide surfaces for microbes, and serve as food for some fish and shrimp. But algae blooms (covering plants, blocking the view, releasing toxins) signal that light, CO₂, nutrients, and flow are out of balance.

Different algae point to different problems: Green algae usually means too much light or excess nutrients; brown algae (diatoms) are common in new tanks or low light; black beard algae (BBA) often results from unstable CO₂ or weak flow; blue-green algae (cyanobacteria) thrive in organic-matter buildup plus low oxygen. Identify the algae type and you can adjust accordingly, rather than blindly dosing algaecides or changing water.

Quick reference

The table below lists likely causes and adjustments by algae color and form. Real situations may involve multiple factors; this is a starting point.

Green algae family

Green algae are the most common, including green water, green dust, hair algae, and clumps. All photosynthesize and prefer strong light and high nutrients.

Green water (planktonic green algae)

Appearance: Water cloudy green, like green tea or matcha. Hold a cup of water to light and see suspended green particles.
Cause: Single-celled planktonic green algae multiplying rapidly. Usually appears when: ① light > 8 hours daily; ② excess N/P (too many fish, overfeeding, or excessive liquid fertilizer); ③ early tank, plants not established, no nutrient competition.

Solution:

• Reduce lighting to 6 hours daily for 1–2 weeks.
• Water change 30–50% to remove planktonic algae and excess nutrients.
• Plant lots of fast-growing species (hornwort, egeria, limnophila) to compete for nutrients.
• Stop or reduce liquid fertilizer (see Plant fertilizer).
• Not recommended: UV sterilizer (treats symptom not cause; nutrient surplus remains); algaecides (may harm plants).

Green dust algae (GDA)

Appearance: Thin green powder coating glass and leaf surfaces, like sprinkled matcha. Easily scraped off, grows back next day.
Cause: Normal phenomenon when light and nutrients are sufficient. Usually appears weeks 2–6 after setup, or after trimming when nutrients release. Not a serious issue, just cosmetic.

Solution:

• Physically remove from glass with scraper or magnet brush (manual work, but most effective).
• Dust on leaves can be left alone—when GDA's natural cycle ends (usually 3–4 weeks), it vanishes. Frequent scraping can prolong the cycle.
• If GDA persists > 6 weeks, check if light too strong (daily > 8 hours) or liquid fertilizer excessive.
• No need to panic: GDA doesn't harm plants, only aesthetics.

Hair algae / Thread algae

Appearance: Long green threads tangled on plant leaves, stems, or substrate surface, like green hair. Soft, breaks easily.
Cause: Strong light + high N/P + insufficient CO₂. Photosynthesis raw materials (light and nutrients) abundant, but CO₂ can't keep up, plants grow slowly, algae grab nutrients.

Solution:

• Reduce light to 6–7 hours daily.
• If no CO₂ system, install one (see How to choose a CO₂ system); if you have one, check CO₂ concentration reaches 20–30 ppm.
• Manual removal: use toothbrush or tweezers to wind up hair algae, pull out and discard.
• Water change 20–30% to reduce N/P buildup (see Plant fertilizer).
• Stock algae-eating organisms (cherry shrimp, Amano shrimp, otocinclus), but effect is limited—don't rely on them to clear it completely.

Brown algae (diatoms)

Appearance: Brown or tan powder film covering glass, substrate, leaves, hardscape. Wipes off easily with finger or scraper but grows back quickly.

Cause:

• New tank cycling (most common): Substrate, hardscape, filter media release silicate (SiO₃²⁻); diatoms use silicate to multiply rapidly. Usually appears weeks 1–3 after setup.
• Insufficient light: In established tanks, if light duration reduced or weaker light installed, plant photosynthesis weakens; diatoms (can survive low light) dominate.
• High silicate in tap water: Some regions have high silicate tap water; diatoms bloom after water changes.

Solution:

• New tank: Patiently wait 2–4 weeks; diatoms naturally subside after silicate depletes. Can scrape glass, but no need to worry about substrate or leaves.
• Insufficient light: Increase lighting to 7–8 hours daily, or install stronger light (see How to choose a suitable light).
• Tap-water issue: Switch to RO or purified water mixed with tap; or add silicate adsorbent (treats symptom not cause).
• Stock algae eaters (otocinclus, snails) to speed cleanup, but won't cure the root cause.

Black beard and staghorn algae

Black beard algae (BBA) and staghorn algae are both red algae (Rhodophyta), appearing black-gray, with extremely strong attachment—the most stubborn algae in planted tanks.

Black beard algae (BBA)

Appearance: Black or dark-gray short tufts like a beard, tightly attached to leaf edges, hardscape, filter outlets. Turns red in alcohol (red-algae trait).

Cause:

• Unstable CO₂: CO₂ concentration swings (high day, low night; or cylinder running low, pressure unstable); plant photosynthesis disrupted, BBA moves in.
• Weak flow: Localized stagnant flow, organic matter settles, BBA attaches and grows. Common in filter dead zones, behind hardscape.
• Organic buildup: Leftover food, dead leaves, fish waste not cleaned; decomposition produces organic carbon, BBA loves organic carbon.

Solution:

• Stabilize CO₂: Ensure CO₂ supply stable all day (20–30 ppm); don't turn on CO₂ only after lights on, or turn off before lights off. Check cylinder pressure regularly, replace when running low.
• Increase flow: Adjust filter outlet direction to cover whole tank; or add wavemaker (see How to choose a filter).
• Physical removal: Trim leaves with BBA (don't feel bad—leaving them spreads it); BBA on hardscape can be brushed off, or remove from tank and soak in 3% hydrogen peroxide 5 minutes (turns white and dies).
• Reduce organic matter: Water change 20–30% weekly; regularly clean substrate (siphon out leftovers); reduce feeding amount.
• Not recommended: Glutaraldehyde algaecide (dosage tricky, may harm plants).

Staghorn algae

Appearance: Gray-green or gray-brown, long hair-like, branching like antlers. Texture harder than BBA.
Cause: Large CO₂ swings + light vs CO₂ mismatch (strong light but little CO₂, or much CO₂ but weak light).
Solution: Same as BBA. Stabilize CO₂ + match light intensity (see How to choose a suitable light) + physical removal.

Blue-green algae (cyanobacteria)

Appearance: Blue-green or dark-green slimy film covering substrate, leaves, hardscape surfaces like a carpet. Obvious fishy odor (like a stagnant ditch). Feels slimy, can peel off in sheets.
Not actually algae: Blue-green algae (Cyanobacteria) are bacteria, not true algae, but called "algae" because they photosynthesize and look like algae.

Cause:

• Organic buildup: Excess leftover food, dead leaves, fish waste decompose into organic carbon and nitrogen; cyano multiplies fast.
• Low-oxygen environment: Weak flow, insufficient aeration, substrate and leaf surfaces lack oxygen; cyano (can survive anaerobic or low-O₂ environments) dominates.
• High phosphate: Phosphate (PO₄³⁻) too high (> 1 ppm); cyano uses phosphate to multiply rapidly.

Solution:

• Physical removal: Siphon out cyano film directly (like vacuuming a carpet). Remove as much as possible; remnants will continue spreading.
• Large water changes: 3–5 consecutive days, change 30–50% water daily to remove organic matter and cyano spores.
• Increase flow and aeration: Boost filter flow; add wavemaker; add air pump (run at night). Get water moving over substrate and leaves, increase dissolved oxygen.
• Reduce organic sources: Cut feeding amount; promptly remove dead leaves; siphon substrate weekly.
• Blackout 3 days: Cover with black cloth for complete darkness 3 days (no lights); cyano dies off in large numbers. But plants also affected (especially high-light species); use with caution.
• Antibiotics: Erythromycin, oxytetracycline can kill cyano (it's bacteria), but destroy nitrification system; not recommended for beginners.

Prevention principles

The root of algae blooms is imbalance—light, CO₂, and nutrients don't match; plants can't fully utilize resources, excess light and nutrients grabbed by algae. Core of algae prevention is maintaining balance + stable supply of all three.

Light: less is better than more

• New tank first 2 weeks: Light 6 hours daily, let plants establish roots before algae get a chance.
• After stable: Low-light tank 6–7 hours; high-light tank 7–8 hours. Don't exceed 8 hours unless CO₂ and nutrients keep up.
• Light intensity must also match: low-light tank (per liter < 30 lumens) reduce fertilizer; high-light tank (> 60 lumens) boost CO₂ and fertilizer (see How to choose a suitable light).

CO₂: stable supply

• Stable concentration: CO₂ concentration all day 20–30 ppm, no swings (low morning, high afternoon). Use bubble counter + timer + solenoid valve to control (see How to choose a CO₂ system).
• Turn on CO₂ 1 hour before lights: Let CO₂ concentration rise first; plants can start photosynthesis as soon as lights on.
• Turn off CO₂ 1 hour before lights off: Avoid nighttime CO₂ buildup too high (fish suffocate).

Nutrients: little-and-often

• Split liquid-fertilizer doses: 2–3 times weekly, each time 1/3 to 1/2 recommended dose; don't dump a week's worth at once (see Plant fertilizer).
• Regular water changes: Change 20–30% water weekly to remove excess nitrate and organic matter. Water changes aren't for "cleaning" but for diluting accumulated nutrients, resetting concentration.
• Control feeding amount: Fish waste is nitrogen source; overfeeding equals fertilizing algae. Feed once daily, amount they finish in 2–3 minutes.

Flow: no dead zones

• Adjust filter outlet to cover whole tank; or add wavemaker.
• Check behind hardscape, dense-plant areas, substrate corners for stagnant flow—these spots easily grow BBA and cyano.

Maintenance habits

• Promptly remove dead leaves: When leaves yellow or melt, trim immediately to avoid decay releasing organic matter.
• Siphon substrate: Every 1–2 weeks use siphon to remove surface leftovers and debris from substrate.
• Check filter: Monthly rinse mechanical filter foam (don't wash bio-media) to keep flow smooth.

Algae are the barometer of water quality—identify the type, find the cause, adjust accordingly, far more effective than blindly using algaecides. For first-month water-quality management, see First month; for balancing light, CO₂, and fertilizer, see How to choose a suitable light, How to choose a CO₂ system, and Plant fertilizer. To preview the finished look, open the aquascape studio and generate an image.

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2026-10-11