Aquarium Algae Types and Fixes: Identify and Treat the 7 Most Common Strains

Every aquarium grows algae at some point, and 90 percent of the time the visible algae is a symptom rather than a problem. The algae itself is signaling that the tank has too much light, too many nutrients, not enough plant biomass, or not enough water flow. Treat only the symptom (scrub it off, dose an algaecide) and it comes back within a week. Identify which species you have, diagnose what is feeding it, and rebalance the underlying conditions, and most algae types disappear permanently. This guide walks through the seven most common algae strains and the realistic fix for each.

How to identify algae correctly

Before any treatment, identify the species. Misdiagnosing green hair algae as BBA leads to spending two weeks on the wrong fix. The diagnostic checklist:

• Color: brown, green, red, black, blue-green
• Texture: dust film, slimy mat, fuzzy tuft, hair-like strands, hard crust
• Location: glass, substrate, plant leaves, hardscape, filter outflow
• Tank age: under 2 months versus over 6 months
• Lighting hours: under 6 hours versus over 8 hours

The five most useful identification photos are saved on planted-tank forums and r/PlantedTank wikis. Match your tank against them before treating.

1. Brown algae (diatoms)

What it looks like: Light brown dust film on glass, sand, plant leaves, and hardscape. Wipes off easily with a finger.

When it appears: Weeks 2 to 8 of a new tank. Sometimes returns in mature tanks fed by silicate-heavy tap water.

Cause: Silicates leaching from new sand, new aquasoil, or source water, feeding diatom species. Sometimes also tied to low light.

Fix: Wait. Diatom blooms self-resolve in 2 to 6 weeks as silicates deplete. Wipe glass weekly during water changes, but do not chase the algae. Adding a few otocinclus catfish or nerite snails speeds the cleanup. Do not blackout, do not change lighting, just wait.

2. Green spot algae (GSA)

What it looks like: Hard, circular green dots on the glass and on slow-growing plant leaves (anubias, java fern). Scrapes off only with a razor blade or magnetic algae scraper.

Cause: Low phosphate. Counterintuitively, the cure is dosing more phosphate, not less. In a low-phosphate tank, plants outcompete algae for it everywhere except the hardiest surfaces, and the hard surfaces grow GSA.

Fix: Dose Seachem Flourish Phosphorus or PhosphorusPMDD to maintain 0.5 to 1.0 ppm phosphate. Scrape off the existing crust with a razor blade. Within 2 to 3 weeks, new growth slows dramatically.

3. Green dust algae (GDA)

What it looks like: Bright green powdery film on glass, growing uniformly across the front and side panels. Easily wiped off but reappears within 24 to 48 hours.

Cause: Imbalance between light intensity and CO2/nutrients in a high-light tank. Often appears after upgrading lighting without upgrading CO2.

Fix: Counterintuitive but effective. Do not wipe the glass for 3 weeks. The GDA grows, matures, sporulates, and naturally falls off in a single mass that you remove with a 50 percent water change. Then bring CO2 up to 30 ppm before re-introducing light at full intensity. If GDA returns, reduce photoperiod from 8 to 7 hours and resume.

4. Black beard algae (BBA)

What it looks like: Black, dark green, or very dark red tufts that look like a 1980s carpet, growing on driftwood, slow-growing plant leaves, and on filter outflow lips. Soft to the touch but firmly attached.

Cause: Fluctuating CO2 in a high-light tank, organic buildup, or low water flow in dead zones. BBA is particularly fond of areas where flow stalls.

Fix: Three-step protocol that works in roughly 90 percent of cases:

1. Stabilize CO2 at 30 ppm with a drop checker, or if low-tech, dose Seachem Excel at 2x recommended dose for 14 days
2. Spot-treat affected hardscape with 3 percent hydrogen peroxide using a syringe (filter off, 5 minutes, then resume flow)
3. Add 5 to 10 Amano shrimp per 20 gallons, or 1 to 2 Siamese algae eaters (the real species Crossocheilus oblongus, not the common “Chinese algae eater” mislabel)

BBA turns pink-white within a week of consistent treatment and falls off within 2 to 3 weeks.

5. Green hair algae (GHA) and string algae

What it looks like: Long green strands clinging to rockwork, substrate, and plant leaves. Slimy when wet, sometimes coats entire surfaces.

Cause: High light plus high nutrients with insufficient plant biomass. Common in tanks with under-stocked plants and over-fed fish.

Fix: Manual removal first (twist around a toothbrush handle and pull out), then address the root cause:

• Reduce lighting to 7 hours daily
• Increase plant biomass with fast growers (hornwort, water sprite, frogbit)
• 30 percent water change weekly until controlled
• Add 5 Amano shrimp per 10 gallons

GHA responds to nutrient rebalancing within 2 to 4 weeks.

6. Cyanobacteria (blue-green algae, BGA)

What it looks like: Slimy red, dark green, or blue-green sheets that peel off surfaces in chunks. Distinctive musty smell. Often grows on the substrate at the front of the tank.

Cause: Low water flow combined with organic buildup, often paired with low nitrate (under 1 ppm). Common in over-cleaned tanks that are simultaneously under-flowed.

Fix: Three-part protocol:

1. Manually remove visible mats, gravel vacuum the substrate, and do a 50 percent water change
2. Improve flow with an additional powerhead or repositioned filter outflow
3. For stubborn cases, use ChemiClean (active ingredient erythromycin) per directions, or do a 4-day full blackout with the tank covered in black plastic

Cyano returns quickly if flow is not improved. Treatment without addressing flow is a temporary fix.

7. Staghorn algae

What it looks like: Gray-green branching strands like deer antlers, growing on plant leaves and filter outflow. Less common than BBA.

Cause: Excess ammonia in a high-light tank, often during a “mini-cycle” after adding fish, or after a filter cleanout that disturbed the bacteria colony.

Fix: Address the underlying ammonia issue first. Test ammonia and nitrite, do back-to-back 30 percent water changes if either is detectable, and reduce feeding for one week. Spot-treat with peroxide. Staghorn does not respond well to algae eaters, the fix is water quality.

The universal algae prevention checklist

After identifying and treating a specific algae, prevent return with the following baseline routine:

• Photoperiod 7 to 8 hours daily, not 10+
• Weekly 25 to 40 percent water change with gravel vacuum
• Feed only what fish consume in 2 minutes
• Maintain plant biomass: 30+ percent of tank substrate planted is the rough threshold
• Run a clean filter (rinse media in tank water monthly)
• Test nitrate and phosphate monthly, keep nitrate 5 to 20 ppm and phosphate 0.05 to 1.0 ppm

The tanks that stay algae-free long term are not the ones with the most aggressive treatments, they are the ones with stable parameters, appropriate light, and enough plant biomass to consume nutrients before algae does. See our aquarium water parameters explained and our methodology for testing protocols.

Frequently asked questions

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