Algae in the aquarium — types, causes and action plan

Algae are a symptom, not a disease. Almost every outbreak has a specific cause — too much light, a missing macronutrient, unstable CO₂, or poor flow. This guide helps you identify what you're looking at and make decisions based on the cause, not on some "golden rule" you found on a forum.

Twelve sections: fundamentals, a visual identification guide, an action plan per type, and emergency measures. Use it as a diagnostic manual — come back when something new shows up on the glass.

1. Why algae appear

Algae compete with plants for the same resources: light, CO₂, macro and micronutrients. When plants grow slowly or can't use available resources, algae fill the niche.

Three actual root causes

1. Imbalanced resources — usually too much light relative to available CO₂ or nutrients. Plants can't keep up; algae can.
2. Unstable CO₂ (in injected tanks) — uneven dosing, a cylinder running out mid-photoperiod, CO₂ turning on too late. BBA and staghorn are classic symptoms.
3. Excess organic matter — leftover food, rotting leaves, dead fish. Phosphate and humic compounds feed algae before plants can use them.

What NOT to believe about algae

Popular myths worth ignoring:

• "Nitrate and phosphate feed algae." They do — but they feed plants too. Macro deficiencies favour algae more than their presence (see: GSA from phosphate deficiency).
• "A fresh tank must have algae." It doesn't have to — but often does. It's a period when the bacterial colony is still stabilising and plant roots aren't strong enough to compete.
• "Reduce dosing to beat algae." Usually makes things worse. Plants weaken; algae spread easier.

2. The golden rule of algae control

Fix the plants and algae fade on their own. Most "anti-algae" interventions amount to cutting light, dosing and feeding — which in practice weakens plants and deepens the problem.

Three things to check before you reach for an algicide:

1. Light: 6–8 h / day. If you're doing more, shorten it.
2. CO₂: stable 25–35 mg/L throughout the photoperiod (injected tanks). Start 1 h before lights on, stop 1 h before lights off.
3. Fertiliser: NO₃ 10–25 mg/L, PO₄ 0.5–2 mg/L, K 10–20 mg/L, Fe 0.1–0.5 mg/L. Dose per method (EI / PPS-Pro / Tropica).

If those three are in order and you're doing 20–30% weekly water changes consistently, the issue usually resolves itself in 2–4 weeks.

3. Algae identification guide

Diatoms (brown slime)

Looks like: brown fuzzy coating on glass, leaves and equipment. Wipes off easily with a finger.

When: first 2–6 weeks of a new tank. Feeds on silicates (SiO₂) from tap water and substrate.

Fix: patience. Once plants start actually growing, diatoms fade on their own. Otocinclus or a couple of Nerite snails speed it up.

Green Spot Algae (GSA)

Looks like: hard green dots on glass and older leaves, especially Anubias. Scraped off with a blade.

Cause: phosphate (PO₄) deficiency. Classic in tanks where hobbyists "avoid PO₄" to starve algae — the opposite happens.

Fix: raise PO₄ to 1–2 mg/L by dosing KH₂PO₄. After 1–2 weeks new GSA stops forming. Old dots need mechanical removal.

Green Dust Algae (GDA)

Looks like: a light green film covering glass and surfaces evenly.

Cause: too much light, often in fresh tanks without a mature micro-environment.

Fix: leave it alone for 3–4 weeks (do NOT wipe). GDA goes through a life cycle and the old algae detach. Then do one thorough clean + 50% water change. Wiping early "interrupts" the cycle and GDA returns faster.

Green Water (Euglena bloom)

Looks like: the water is pea-soup green, visibility drops to a few centimeters. Single-cell algae floating in the water column.

Cause: excess light + nitrate + PO₄. Typical after a fish dies or from over-feeding.

Fix: 3-day blackout (completely cover the tank) + fine filter pad, or a UV sterilizer (fastest option). Follow with a big water change.

Black Brush / Beard Algae (BBA)

Looks like: dark green to black tufts on leaf edges, driftwood, filter outlets.

Cause: unstable CO₂ (most common), poor water flow, high PO₄ with low NO₃.

Fix: dial in CO₂ to a stable 25–35 mg/L throughout the photoperiod, increase filter flow, balance N:P (15–25 NO₃ to 0.5–2 PO₄). Mechanical: spot-dose 3% H₂O₂ from a pipette with the filter off.

Staghorn algae

Looks like: grey-green branched tufts resembling deer antlers, on leaf edges and equipment.

Cause: similar to BBA — imbalanced CO₂ and NO₃. Often appears alongside BBA.

Fix: same as for BBA. H₂O₂ works especially well (3% dosed directly on tufts — max 1 ml per 10 L tank).

Hair / Thread Algae

Looks like: long green "strings" (1–10 cm) attached to leaves and equipment.

Cause: excess light + iron, often in densely planted tanks. Sometimes spreads after a "glut" of dosing.

Fix: shorten photoperiod to 6 h, pause Fe dosing for a week. Mechanically — wind threads onto a stick or toothbrush. Amano shrimp and Neocaridina eat it.

Blue-Green Algae / Cyanobacteria (BGA)

Looks like: dark green slimy "mat" covering large areas, usually substrate or under the filter housing. Smells strongly of mud.

Important: these aren't real algae — they're photosynthetic bacteria. Regular algicides don't work.

Cause:poor water flow (dead zones), low NO₃ (< 5 mg/L), excess organic matter.

Fix:

1. 3-day blackout (BGA dies without light).
2. Raise NO₃ to 15–20 mg/L (dose KNO₃).
3. Check flow — you may need a second filter outlet.
4. If it returns — 4-day erythromycin course (1 tablet 500 mg / 200 L daily). Watch your biofilter.

Fuzz algae

Looks like: short (1–3 mm), light green "fuzz" on leaves and decorations. Especially on young leaves.

Cause: young tank, plants not fully functional yet.

Fix: patience. Once plants settle in, fuzz disappears. Otocinclus and Amano shrimp help.

4. The action plan — what to do in what order

Regardless of algae type, the order of steps is always the same:

1. Identify the algae — section 3 above. Without identification you're guessing.
2. Measure parameters: NO₃, PO₄, pH, KH, temperature. Note them. That's your baseline.
3. Fix the obvious: photoperiod too long, weak flow, unbalanced dosing.
4. Manual removal — scrape, pull, prune what you can. Don't leave a pile of algae in the tank "out of spite".
5. Big water change (50%) — resets organic compounds and algae spores.
6. Wait 2–3 weeks. Algae don't vanish overnight, but they should stop spreading.
7. If after 3 weeks there's no progress — chemical measures (H₂O₂, algicides, UV) as a last resort.

5. Prevention — habits that keep algae at bay

Most algae problems can be prevented by a few consistent habits:

Photoperiod

• 6–8 h daily. More only in CO₂-injected, heavily fertilised tanks.
• Timer on the outlet — eliminates "forgetting".
• One long photoperiod rather than two short ones with a break (siesta works, but adds complexity).

Water changes and substrate vacuuming

• 20–30% weekly, without skipping. See our water change frequency guide.
• Vacuum the substrate where food collects (under the filter outflow, around roots).
• Remove dying leaves before they rot — that's the main source of sudden phosphate spikes.

Feeding

• Once a day, only what fish finish in 2 minutes.
• One "fasting day" per week — doesn't hurt fish, cuts the organic load.
• Higher-quality food (less ash = less waste).

Stocking and plants

• Don't overstock beyond the recommended load.
• A densely planted tank = fewer algae (plants out-compete them).
• Fast-growing floaters (hornwort, pistia) are excellent nitrate "sponges" during the maturation phase.

6. Who eats what — algae eaters

Algae eaters are a supplement, not a replacement for diagnosis. But the right ones help keep things tidy.

Nothing eats BGA (cyanobacteria). Not shrimp, not snails, not fish. They're bacteria, not algae.

7. Emergency measures — when manual isn't enough

Blackout (3–4 days)

Completely cover the tank with a blanket or cardboard for 3–4 days. Fish and plants survive; algae — especially green water and BGA — die.

• Before: 50% water change.
• During: filter runs, heater runs, light off, tank stays covered.
• After: 50% water change, re-introduce light gradually (4 h day 1, 6 h day 2).

Don't go longer than 4 days — sensitive plants may start dying.

Hydrogen peroxide (H₂O₂)

Pharmacy-grade 3% H₂O₂ dosed directly from a pipette onto BBA, staghorn, GSA. Dose: max 1 ml per 10 L tank daily.

• Turn off filter for 15 min after dosing (so it acts locally).
• Dose directly onto algae, not into the water column.
• After 3–4 days of dosing, algae turn black and fall off.
• Kills some plants (Rotala wallichii, Riccia). Test on a single leaf first.

UV filter

Fastest fix for green water and waterborne bacteria. UV kills microorganisms as they pass through the lamp.

• For 100 L: 5–9 W is enough.
• Run 24–48 h and you can turn it off.
• Doesn't work on algae attached to surfaces (BBA, GSA etc.).

Chemical algicides (last resort)

Easy-Life AlgExit, Seachem Flourish Excel (low dose), API Algaefix. Effective, but:

• Can hurt sensitive plants (Vallisneria, Elodea).
• Toxic to shrimp — don't use in shrimp tanks.
• Treats the symptom, not the cause — algae return if conditions don't change.

8. When to give up and rescape

Sometimes the fight isn't worth it. Consider a reset if:

• BBA covers 50%+ of plants and equipment despite 2 months of CO₂ and dosing tweaks.
• BGA returns after every erythromycin course.
• The tank is in "slow collapse" — plants dying, algae growing, nothing helps.
• Mental fatigue — fishkeeping is supposed to be fun.

Rescape: take everything out (fish into a bucket of tank water, plants cleaned of algae, filter and media preserved), toss the substrate (especially if BGA), clean the glass, refill. Preserving the filter means you don't have to cycle from scratch.

9. FAQ

Why does my new tank have algae?

The first 6–8 weeks are the "algae window" — plants don't have mature roots yet, bacteria aren't fully active. Diatoms and GDA are normal. Don't panic, don't break the cycle, don't up dosing in reaction.

Dim the lights or shorten the photoperiod?

Shorten the photoperiod. Dimming (timer ramp, dimmer) rarely helps — plants don't grow, algae adapt to weaker light.

Do algae hurt fish?

Algae themselves — no. But BGA produces toxins that can hurt shrimp. Euglena (green water) is usually safe but drastically reduces visibility and plant photosynthesis.

I bought an algicide and algae came back after 2 weeks.

Because you didn't fix the cause. The algicide killed existing algae, but conditions (excess light, unstable CO₂, macro deficiency) still favour regrowth. Diagnose the environment, not just the symptom.

How much algae is "too much"?

Up to 10% coverage on leaves and glass is normal. Above — time to act. A completely algae-free tank usually means excessive hygiene (algicides, UV), which brings its own problems.

Can heavy EI dosing "beat" algae?

Paradoxically yes. The Estimative Index method (large macro doses + 50% weekly water change) works by letting plants outpace algae. But it needs CO₂ and strong light — without those, EI is just expensive fertilising.

I have green water and don't want to do a blackout. What else?

UV filter (5–9 W for 100 L) — 24–48 h and the water is crystal clear. The only method faster than a blackout.

Wrap-up

Algae are a signal to diagnose, not panic. Start with identification, measure parameters, fix the obvious (photoperiod, CO₂, dosing) and give it 2–3 weeks. Mechanical removal (scraping, pruning, siphoning) is fine; chemical is last resort.

The best anti-algae tank is stable, densely planted, consistently fertilised, with a reasonable photoperiod and regular water changes. Nail those basics and algae will stay at trace levels — which is exactly how it should be.

Related: complete water parameters guide — if you don't know your NO₃, PO₄ and CO₂, you don't know what to fix.