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Worst case: Act within days3 possible causes

Green water

The water column itself is green and opaque, rather than algae growing on surfaces.

Short answer

Green water is a bloom of free-floating single-celled algae. It is triggered by excess light plus a nutrient release, and it does not respond to water changes because the algae reproduce faster than you can dilute them.

Check these first, in this order

  1. Check for direct sunlight on the tank
  2. Check the photoperiod
  3. Consider whether the substrate was recently disturbed

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.

Excess light, often direct sunlight

Act within daysvery common

How to tell: The tank is near a window, or the photoperiod is over about nine hours.

Check

  • Position relative to windows
  • Timer settings

Response

  • Shade the tank
  • Reduce photoperiod to six to eight hours
  • Blackout or UV as below

Nutrient release after substrate disturbance

Act within dayscommon

How to tell: Followed a deep substrate vacuum, replanting or a rescape.

Check

  • Recent substrate work
  • Ammonia reading

Response

  • Blackout treatment
  • Reduce feeding
  • Add fast-growing plants

Immature tank

Observe and monitorcommon

How to tell: Tank is under about three months old and otherwise stable.

Check

  • Tank age

Response

  • Blackout or UV, then let the tank mature
  • Avoid over-correcting nutrients

What not to do

  • Repeated large water changes, which remove nutrients slower than the algae reproduce and can make it worse by improving conditions.
  • Algaecides in a tank containing invertebrates.

Prevention

  • Keep the tank out of direct sunlight
  • Photoperiod of six to eight hours
  • Maintain fast-growing plants