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Worst case: Act within days4 possible causes

Algae is taking over

Algae is growing faster than it can be removed, on glass, hardscape, plant leaves or all three.

Short answer

Algae grows where light exceeds what the plants can use with the carbon and nutrients available. The type of algae narrows the cause; reducing light and improving flow and maintenance fixes most cases.

Check these first, in this order

  1. Identify the type — diatoms, green spot, hair, black brush and cyanobacteria have different causes.
  2. Check the photoperiod and whether the tank gets direct sun.
  3. Check nitrate and how long since the last water change.
  4. Check flow, and whether there are dead spots.

Possible causes

Ordered by urgency, then by how commonly each one turns out to be the answer. The discriminator is the observation that separates this cause from the others — use it before acting.

Too much light for the plant mass

Act within daysvery common

How to tell: Long photoperiod, bright light, and few or slow-growing plants. Green spot and green dust algae dominate.

Check

  • Photoperiod length
  • Whether plants are actively growing

Response

  • Reduce photoperiod to six to eight hours
  • Reduce intensity if the light is dimmable
  • Add fast-growing plants

Unstable or insufficient CO₂ in a high-light planted tank

Act within dayscommon

How to tell: Black brush algae on leaf edges and hardscape, in a tank with injected CO₂ and strong light.

Check

  • Drop checker colour through the day
  • Whether injection starts before lights-on

Response

  • Stabilise injection and start it an hour or two before the lights
  • Increase flow to distribute it evenly
  • Trim and remove affected leaves

Accumulated organics and poor flow

Act within dayscommon

How to tell: Slimy dark blue-green sheets with a distinctive earthy smell — this is cyanobacteria, not an alga — usually in low-flow corners.

Check

  • Smell
  • Whether it peels off in sheets
  • Flow in the affected area

Response

  • Physically remove as much as possible, improve flow into the area, and increase water changes.
  • A blackout of several days is effective; treat the flow and organics problem or it returns.

New tank — diatoms

No action neededvery common

How to tell: Brown dusty film on glass and leaves in a tank under about three months old. Wipes off easily.

Check

  • Tank age
  • Whether it wipes away cleanly

Response

  • Wipe the glass and wait. It resolves as the tank matures.
  • Otocinclus and nerite snails graze it readily.

What not to do

  • Buying an algae-eating fish as a first response. Several of the popular choices outgrow the tank or turn aggressive.
  • Increasing light to help plants outcompete algae. It does the opposite.

Prevention

  • Short photoperiod
  • Plant heavily from the start
  • Regular maintenance
  • Good circulation