Every planted or partially-planted tank grows algae. The question isn’t how to prevent all algae (impossible) but how to keep it manageable and how to fix it when it takes over.

Aquarium algae comes in seven main types, and they respond to different treatments. Blasting the tank with algaecide is the wrong answer for most of them — often it kills your fish and plants while barely denting the algae. This guide walks you through identification and the actual fix for each.

Diagnose which type you have

Different algae look different. Start here:

• Green film on glass — common green spot algae. Normal. Just wipe.
• Water itself is green — green water (single-celled floating algae). See section 1.
• Dark green/black tufts on plants and decor — black beard algae (BBA). See section 2.
• Brown dusty coating on everything, especially in new tanks — diatoms. See section 3.
• Bright green thread-like strands — hair or thread algae. See section 4.
• Stiff grey/dark green strands like a mini shrub — staghorn algae. See section 5.
• Slimy blue-green or blackish sheets that peel off — cyanobacteria (“blue-green algae”). See section 6. Not technically algae but treated together.
• Green film that’s easy to wipe — green spot algae. See section 7.

The universal fixes (do these first)

Before treating any specific algae type, apply these baselines. They fix 60% of problems on their own.

• Reduce lighting. Cut photoperiod to 6–7 hours. Move tank away from any window.
• Increase water changes. Two 25% weekly changes if you’re currently doing one.
• Reduce feeding. Overfeeding = more waste = more nitrate = more algae food.
• Add more real plants. Fast-growing stems (hornwort, hygrophila, ludwigia) outcompete algae for nutrients.
• Test water: API Master Test Kit. Nitrate over 40 ppm feeds algae; get it down through water changes and better stocking.

1. Green water (free-floating single-cell algae)

Look: water itself is green, like weak pea soup. May glow under bright light.

Cause: too much light + high nutrients. Common in south-facing rooms.

Fixes:

• Blackout treatment: cover tank completely with towels for 3 days. No light. Fish are fine. Follow with 50% water change. Kills 90%+ of the algae.
• UV steriliser — attach an aquarium UV steriliser to your filter output. Kills free-floating algae as water passes through the UV chamber. £25–£60. Very effective for green water specifically.
• Long-term: reduce photoperiod, add floating plants (Amazon frogbit is excellent), do more frequent water changes.

2. Black beard algae (BBA)

Look: dark green to black tufts, feels like a stiff toothbrush, attaches to plant leaves and decor.

Cause: unstable CO2, high phosphate, low water flow, or old plants shedding.

Fixes:

• Spot-treat with liquid carbon: dose Easy Life Easycarbo or Seachem Excel directly onto BBA tufts with a syringe (lights off, filter briefly off). BBA turns red within a day, then dies. Do this weekly for 3–4 weeks.
• Add Siamese algae eaters (Crossocheilus siamensis) — one of the very few fish that eat BBA. Get true SAEs, not lookalikes; independent aquatic shops usually stock them.
• Improve water flow. BBA loves stagnant spots.
• Stabilise CO2 if you inject pressurised CO2 (see our DIY CO2 guide for beginners).
• Prune infected leaves rather than trying to save them. Regrowth without BBA is possible.

3. Brown diatoms

Look: brown/rust-coloured dusty coating on glass, substrate, plants, and decor. Feels like wet dust.

Cause: silicates in new tank water. Very common in the first 2–8 weeks of a new tank.

Fixes:

• Ignore. Diatoms in a new tank are a phase. They peak around week 3–6 and then disappear as silicates are consumed.
• Otocinclus catfish eat diatoms voraciously. Add 4–6 to a tank over 60L that’s been cycled at least 6 weeks.
• Nerite snails eat diatoms without breeding in freshwater — perfect algae cleanup.
• Wipe glass with aquarium magnet cleaner weekly during the phase.

Persistent diatoms after 8 weeks usually means silicate is entering from tap water. RO water for water changes solves it.

4. Hair / thread algae

Look: bright green fine strands, like green cotton wool, especially on plants and hardscape.

Cause: excess iron, excess light, or nutrient imbalance in planted tanks.

Fixes:

• Physical removal. Twist a toothbrush or a chopstick in the algae and it winds around like spaghetti on a fork. Remove weekly.
• Amano shrimp — Caridina multidentata eat hair algae specifically. 5–10 per 100L.
• Reduce iron fertiliser if you dose ferts.
• Reduce photoperiod to 6 hours.
• Rosy barbs or American flag fish eat hair algae aggressively but are aggressive with other fish; only for tanks stocked with similarly hardy fish.

5. Staghorn algae

Look: stiff grey-green branching strands, like antlers or a tiny shrub. Attached to plant edges and decor.

Cause: poor water quality, high ammonia, unstable CO2. Common in overstocked tanks.

Fixes:

• Big water change (50%), test water, fix underlying cause.
• Liquid carbon spot treatment (Easy Life Easycarbo or Seachem Excel) — same as BBA treatment.
• Improve filtration. Under-filtered tanks develop staghorn.

6. Cyanobacteria (blue-green algae)

Look: slimy blue-green, dark green, or blackish sheets that peel off in one piece. Smells swampy.

Cause: poor water flow + high organics. Often develops in areas of stagnant water.

Fixes:

• Physically siphon out all visible cyanobacteria. It comes off in sheets.
• 3-day blackout: cover tank completely with towels, no light. Cyanobacteria dies. Follow with 50% water change.
• Improve water flow. Cyanobacteria only grows in stagnant spots.
• Increase nitrate — counterintuitive but cyanobacteria thrives specifically in low-nitrate high-organics conditions. Occasionally the cause is over-successful nitrate control.
• If persistent: erythromycin-based aquarium antibiotics kill cyanobacteria specifically. Use as last resort.

7. Green spot algae

Look: small dark green dots on glass and slow-growing plant leaves (Anubias, Java fern).

Cause: low phosphate in planted tanks.

Fixes:

• Wipe glass with an aquarium magnet cleaner or algae scraper weekly. Normal maintenance.
• Nerite snails eat green spot happily.
• In planted tanks: add a small phosphate supplement. Green spot algae specifically indicates phosphate deficiency in demanding planted tanks.

Algae eaters — what actually works in the UK

Species	Eats	Notes
Otocinclus catfish	Diatoms, soft green film	Fragile, needs mature tank
Bristlenose pleco	Green algae, biofilm	Excellent, hardy, see our care guide
Amano shrimp	Hair algae, biofilm	Shrimp-safe tanks only
Nerite snails	Diatoms, green spot, soft algae	Won’t breed in freshwater — perfect
Siamese algae eater	Black beard algae	Get true SAE, not lookalikes
Ramshorn snails	Biofilm, dying leaves	Breed prolifically — mixed blessing

What DOESN’T work in UK community tanks: Chinese algae eaters (become aggressive as adults, don’t eat much algae), plecs bigger than bristlenose (grow too big for most tanks).

Prevention pattern

Most algae problems come from three root causes:

1. Too much light — reduce photoperiod to 6–8 hours; move tank away from windows
2. Too many nutrients — reduce feeding; do bigger water changes; understock
3. Not enough real plants competing — add fast-growing plants like hornwort, wisteria, water sprite

Tanks that never battle algae usually have: heavily planted setups with fast-growing stems, moderate lighting, moderate stocking, weekly water changes, and 3–5 nerite snails as low-maintenance cleanup.

What to avoid

• Algaecide products (excluding liquid carbon spot-treatment). Broad-spectrum aquarium algaecides usually harm fish and plants along with the algae. Rarely worth it.
• Cleaning ALL glass at once. Leave the back wall alone — the algae there is a nitrate sink and food source for grazing fish and shrimp.
• Copper-based treatments in shrimp/snail tanks.
• Full water changes. Removes beneficial bacteria alongside algae. Rarely fixes the underlying cause.

Bottom line

Algae is a symptom, not a disease. Identify the type by look, treat the specific cause, and add appropriate cleanup crew. Bristlenose plecos + a few nerite snails + weekly water changes prevent most problems from ever developing.

Green water gets a blackout or a UV steriliser. Black beard gets liquid carbon spot treatment. Diatoms just need patience and otos. Everything else responds to the universal fixes: less light, more plants, less feeding, more water changes.

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Related: how to cycle a fish tank, why is my fish tank cloudy, bristlenose pleco care — plecs are the best all-round UK community-tank algae eater.

— Jaqline Smalling, FishForever