Types of Aquarium Algae: How to Identify and Fix Each One

Published 3 June 2026

Algae is the most universal frustration in the planted tank hobby. Every aquarium grows some algae — it’s a natural part of any aquatic ecosystem. The goal isn’t elimination; it’s control. And effective control starts with identifying which type you’re dealing with, because the causes and solutions differ significantly between types.

Why algae appears: the basics

All algae growth comes down to an imbalance — usually too much light, too many nutrients, or both, relative to the amount of healthy plant growth in the tank.

In a well-balanced planted tank, the plants absorb available nutrients and compete with algae for light. When that balance is disrupted — by too much light, excess nutrients from overfeeding or insufficient water changes, or a new tank without established plant growth — algae fills the gap.

This is why troubleshooting algae always involves looking at three variables: light intensity, photoperiod, and nutrient load. Solving the root cause is the only lasting fix. Chemical algaecides treat symptoms; they don’t address what caused the bloom.

The most common aquarium algae types

Green spot algae (GSA)

What it looks like: Small, hard, dark green spots on the glass and slow-growing leaves (especially anubias). Unlike film algae, these spots can’t be wiped off with a finger — they require a razor blade or hard scraper.

Cause: Usually indicates low phosphate in the water column. Counterintuitively, GSA is often a sign that nutrient levels are too low, not too high. In a low-tech planted tank with very pure water, sporadic GSA on the glass is common and mostly cosmetic.

Fix: If GSA is limited to the glass, don’t worry — scrape it during your weekly maintenance. If it’s spreading aggressively onto plant leaves, test phosphate levels and consider a small dose of a phosphate-containing fertiliser.

Green dust algae (GDA)

What it looks like: A thin, dusty green film on the glass that wipes off easily with a finger or magnetic scraper. Appears within days of a water change.

Cause: Common in new tanks and tanks with high light. Often self-limiting — GDA typically peaks then diminishes as the tank establishes.

Fix: Wipe the glass during water changes. Reduce photoperiod by 1 hour if it’s persistent. GDA that disappears and reappears weekly is a cosmetic nuisance, not a serious problem.

Brown algae (diatoms)

What it looks like: A soft, brown, dusty coating on the glass, substrate, plants, and decorations. Wipes off easily. Extremely common in new tanks.

Cause: Diatoms thrive on silicates, which are abundant in new substrates, new rocks, and fresh tap water. They’re nearly universal in newly set-up tanks during the first 2–8 weeks.

Fix: Wait. Diatoms are self-limiting — as silicate levels drop and the tank matures, they crash on their own. Otocinclus catfish, nerite snails, and amano shrimp will consume diatoms enthusiastically if you want to speed the process. Don’t scrub everything obsessively — let the tank mature.

Hair algae / thread algae

What it looks like: Long, fine, green threads growing from the substrate, plant leaves, and decorations. Can form thick mats. The individual strands range from a few millimetres to several centimetres.

Cause: Usually a combination of high light and high nutrients. Very common in new tanks that are well-lit before plant growth is established. Also triggered by high iron or nitrogen levels.

Fix:

1. Reduce photoperiod by 1–2 hours
2. Reduce light intensity if adjustable
3. Add fast-growing plants (hornwort, water sprite, floating plants) to compete for nutrients
4. Manual removal: twist it around an old toothbrush or your finger and pull it out in clumps
5. Amano shrimp eat hair algae voraciously — 5–10 in a 40L tank makes a visible difference within weeks

Black beard algae (BBA)

What it looks like: Dark grey to black tufts, 1–3mm long, growing on plant leaves, hardscape, and filter outputs. Extremely tough — does not wipe off. Looks like a miniature beard. One of the most stubborn algae types in the hobby.

Cause: BBA (Audouinella sp., also called staghorn algae when it grows as single branches) is associated with fluctuating CO2 levels, high organic load, and areas of low or inconsistent flow. In low-tech tanks it often appears at filter outputs, on slow-growing leaves, and on hardscape.

Fix:

• Target with Excel (liquid carbon): Seachem Excel applied directly to BBA with a syringe (during a water change with tank level lowered) turns it pink within 24 hours and kills it within a week. This is the most effective low-tech treatment.
• Improve flow: BBA frequently colonises dead-spot areas. Adjusting filter positioning to improve circulation in affected areas helps prevent regrowth.
• Reduce organic load: increase water change frequency; reduce feeding.
• Siamese algae eaters (true SAE, Crossocheilus oblongus) are one of the few fish that eat BBA — though adults become less interested over time.

Staghorn algae

What it looks like: Single-branched grey or green strands, 1–3cm long, growing in a stag-horn pattern from plant edges and hardscape. Similar to BBA but lighter in colour.

Cause: Similar to BBA — associated with low CO2, fluctuating CO2 levels, and areas of low flow.

Fix: Same approach as BBA — Excel spot treatment, improved flow, reduced organic load.

Cyanobacteria (blue-green algae)

What it looks like: A thick, slimy, blue-green or red-purple coating that spreads in sheets across the substrate, plants, and decorations. Has a distinctive earthy, unpleasant smell. Despite the name and appearance, it’s not true algae — it’s a photosynthetic bacteria.

Cause: Most commonly associated with low nitrates, poor circulation, and organic matter accumulation in the substrate. New tanks with disturbed substrate are particularly vulnerable.

Fix:

1. Manually remove as much as possible (gloves recommended; it can irritate skin)
2. Improve circulation — direct filter output towards affected areas
3. Do a 3-day blackout (cover the tank completely) — this is often effective for mild outbreaks
4. Check nitrate levels — if very low, the tank may need nutrient supplementation
5. For persistent outbreaks: a course of erythromycin is effective but kills beneficial bacteria simultaneously; use only as a last resort

Green water (algae bloom)

What it looks like: The entire water column turns green, sometimes very opaque. The individual algae cells (euglenoids) are microscopic — you see them as a colour, not individual strands.

Cause: Typically direct sunlight or very high light combined with excess nutrients. Common in new tanks that are positioned near windows.

Fix:

• Remove any direct sunlight exposure immediately
• Reduce photoperiod to 4–5 hours temporarily
• 30–40% water change to dilute nutrients
• 2–3 day blackout for severe cases
• A UV steriliser eliminates green water completely but is an additional expense

See our dedicated guide: Why is my aquarium water cloudy?

Prevention: the best long-term strategy

Consistent algae prevention comes down to a few habits:

• Timer on the light — 6–8 hours maximum in a low-tech tank, consistent every day
• Regular water changes — weekly 20–25% reduces nutrient accumulation
• Don’t overfeed — uneaten food decays and fuels algae
• Healthy plant mass — a well-planted tank competes with algae naturally. Dense floating plants and fast-growing stems absorb nutrients that would otherwise feed algae
• Introduce algae-eating crew — nerite snails, amano shrimp, otocinclus, and (for hair algae) Siamese algae eaters provide consistent grazing

The planted tank community’s saying: “the solution to algae is more plants.” There’s genuine truth to it.

For more on setting up a balanced low-tech planted tank, see our complete beginner’s guide.