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Green Water

Green Water (Phytoplankton Bloom)

Scientific name: Euglena viridis, Chlorella sp., Chlamydomonas sp. Taxonomic group: Unicellular euglenophytes and chlorophytes in suspension Water: Mainly freshwater (rarely saltwater) Color: Deep green, yellowish-green, opaque water Clarity: The water is cloudy; in severe cases, fish cannot be seen from a distance of 5 cm

Appearance: The water turns completely opaque green. There is no visible algae on the glass or decorations—the color comes from millions of microalgae suspended in the water itself. This can develop within 24–48 hours if conditions are ideal.

Where does it appear?: The entire volume of water in the aquarium.

Is it dangerous?:

  • Not directly for fish
  • Reduces visibility
  • It consumes large amounts of O₂ at night—which can cause nocturnal hypoxia and deaths
  • In large clusters exposed to direct sunlight, it can be dangerous

Related parameters:

  • Direct sunlight: The most common cause
  • NH₃/NH₄⁺: High (new aquarium, overfeeding)
  • NO₃: > 50 ppm
  • PO₄: High
  • High temperatures: Summer, unmonitored aquariums

Main causes:

  1. Direct sunlight on the aquarium — the most common
  2. Very high nutrient levels (NH₃, NO₃, PO₄)
  3. New aquarium without bacteria that consume nutrients
  4. High temperature combined with bright light
  5. Absence of competing plants

How to remove it:

  • UV Sterilizer: The most effective and fastest method. A 5–9W UV sterilizer connected to the filtration system kills microalgae in 2–4 days. Then perform a water change.
  • Total darkness: 3–5 days without any light. Very effective. The plants will survive, and the fish won't suffer.
  • Flocculant: Some products (Seachem Clarity, JBL Clearol) cause microalgae to flocculate so they can be trapped by the filter. The filter should be lined with fine wool or cotton.
  • Massive water changes: Drastically reduce nutrients. Change 50% of the water daily for 3–5 days.
  • Keep out of direct sunlight: Move the aquarium or cover the window.
  • Biological: Daphnia (Daphnia magna) — they feed on microalgae very efficiently. Introduce a culture of daphnia for 1 week.

Prevention: Never expose the aquarium to direct sunlight; keep nutrient levels within the proper range; and avoid fast-growing, competing plants.