Why does the algae come back after I clean it?

Because cleaning removes the symptom. Algae that returns within days usually means the cause is untouched: unstable CO₂, a photoperiod that is too long, a dead flow zone, or accumulating organic waste in the filter.

How long should the light be on in a planted tank?

Six to eight hours is the usual working range. Start at six, raise it only if plants are clearly light-starved, and keep the schedule identical every day — algae respond to inconsistency more than to any single number.

What is the fastest way to get rid of algae?

Physically remove what you can, then fix the imbalance in the same week. For green water and cyanobacteria a 3–5 day blackout works quickly; for black beard and staghorn, no treatment sticks unless CO₂ stability and flow improve.

Guide

Aquarium algae identification

Q: What algae grows on slow-growing plant leaves?

Black beard algae and staghorn algae are the usual suspects on slow growers such as Anubias, Bucephalandra and Java fern. Their leaves are not replacing tissue quickly, so any film or tuft that settles stays visible — which is why these plants are the first place an imbalance shows up.

Q: Is blue-green algae really an algae?

No. Blue-green algae is cyanobacteria, a bacterium. That is why it forms slimy sheets that smell earthy and why light-and-nutrient rebalancing works on it where it would do little for true algae.

Ten algae cover almost every case in a freshwater tank, and each one points at a different imbalance: black beard and staghorn at unstable CO₂ and weak flow, green spot at low phosphate, cyanobacteria at low nitrate and dead spots, diatoms at a young tank. Match the look first, then fix the cause — cleaning alone brings it back within a week.

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AquaAI · scanningIwagumi 60P

Staghorn algae 92%

Rotala · healthy

Low flow zone

3 findingsCause: unstable CO₂ + organics

Run CO₂ on a fixed timer, on 1 h before lights
Point filter return at the back-left corner
Trim affected tips · clean filter in tank water

Black beard algae (BBA)

Dark, stiff tufts on slow-growing leaves, rock, wood and filter outlets.

Usual causeUnstable CO₂ — on late, off early, or swinging between days — plus a dead flow zone where organics settle.

FixRemove affected leaves, run CO₂ on a timer that never changes, add flow across the affected area, clean the filter and cut feeding. Spot treatments only buy time.

Staghorn algae

Grey-green branching strands, like tiny antlers, usually on leaf edges and equipment.

Usual causeLow CO₂ relative to light, a new-tank ammonia bump, or dirty filter media.

FixTwirl it out, restore CO₂ stability, clean the media in tank water, and reduce light intensity before touching fertilizers.

Green spot algae (GSA)

Hard green dots on the glass and on slow leaves — you scrape it, it comes back.

Usual causePhosphate too low relative to light and CO₂, or an old tube that has drifted spectrum.

FixScrape the glass, bring phosphate back to a detectable level, stabilise CO₂ and shorten the photoperiod slightly.

Green dust algae

A soft green film on the glass that wipes off as green dust in the water.

Usual causeA young, still-maturing tank with high light.

FixWipe before water changes, keep the schedule steady and wait — this one usually fades as the tank matures.

Hair / thread algae

Long, fine green threads wrapping plant stems and moss.

Usual causeNutrient imbalance with too much light and soft CO₂ — growth outpaces the plants' uptake.

FixTwirl out with a toothbrush, cut the photoperiod, keep fertilization consistent rather than "extra".

Cladophora

Coarse, woolly dark-green clumps that feel like steel wool and survive scrubbing.

Usual causeSpores carried in with plants, thriving on accumulated organics.

FixRemove aggressively and never let fragments drift into the tank — it spreads by breakage. Long-term: keep organics down.

Cyanobacteria (blue-green algae)

Slimy blue-green or red-brown sheets over substrate and plants, with an earthy smell.

Usual causeLow nitrate, low flow and dead spots — not an algae at all, a bacterium.

FixSiphon it out, raise nitrate to a readable level, fix flow, and run a 3-day blackout. Antibiotic treatments are a last resort, only where legal.

Brown diatom algae

Brown dust on glass, leaves and substrate — easy to wipe, comes back fast.

Usual causeSilicates and an immature bacterial cycle; very common in the first weeks of a new tank.

FixWipe and change water, add a few otocinclus or nerite snails if appropriate, and let the tank mature.

Green water

Suspended single-celled algae — the tank turns pea-soup green, nothing is sharp.

Usual causeAmmonia plus light in a new or overfed tank.

FixA 3–5 day blackout with a blanket over the tank, water changes before and after, cut feeding, and a UV steriliser if it returns.

Rhizoclonium

Pale, stringy green strands that gather in high-flow areas and around filter outlets.

Usual causeLow nutrients and CO₂ delivered into strong flow — the plant cannot outgrow it.

FixFeed the plants properly, stabilise CO₂, remove the strands by hand and keep flow from blasting one spot.

The pattern behind all ten

Almost every algae problem resolves to one of four levers. Fixing the lever is slower than scraping the glass and lasts longer:

Light — 6–8 hours, the same every day, and no sunlight from a window.
CO₂ — on before lights, off before lights out, never drifting. Test with pH and kH and keep the estimate in the 15–30 ppm range.
Nutrients — enough nitrate and phosphate that plants are never the limiting factor; too little causes as many algae as too much.
Flow and organics — no dead spots, filter cleaned in tank water, and a water change that matches your net volume.

What actually does not work

Cutting all fertilizer at the first sign of algae. Starving plants hands the advantage to algae, which need far less.
A total blackout every month. It resets the symptom and stresses the plants you are trying to grow.
Chemical "algae removers" as a routine. They knock the population down, then the same imbalance grows it back — with interest.
Buying an algae crew first. Snails and otocinclus eat the evidence, not the cause.

Questions

What algae grows on slow-growing plant leaves?

Black beard and staghorn, usually. Anubias, Bucephalandra and Java fern replace tissue slowly, so anything that settles stays visible — they are the tank's early-warning system.

Is blue-green algae really an algae?

No, it is cyanobacteria. That is why slimy sheets plus an earthy smell is the tell, and why nitrate, flow and a blackout work on it.

Why does it come back after cleaning?

The cause was never touched: unstable CO₂, too long a photoperiod, a dead flow zone, or waste in the filter.

How long should the light be on?

Six to eight hours, identical every day. Consistency matters more than the exact number.

What is the fastest fix?

Remove what you can by hand, then correct the imbalance in the same week. Green water and cyanobacteria respond to a 3–5 day blackout; black beard does not.

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