Maintenance 5 min read

Green Water Aquarium Guide

By The Aquarium Calculators Editorial Team ·

Green water is a free-floating algae bloom that turns tank water pea-soup opaque. Find out what triggers it and the fastest proven fixes -- UV sterilizer and blackout.

Aquarium with pea soup green water algae bloom obscuring fish

Quick Answer

Green water is caused by a bloom of free-floating single-celled algae suspended throughout the water column, usually triggered by direct sunlight combined with excess nutrients. A UV sterilizer is the most reliable fix; a 3-4 day total blackout also works if you don't have one.

Key Takeaways

  • Green water is a free-floating algae bloom suspended in the water column, not a surface growth
  • Direct sunlight combined with excess nutrients is the most common trigger
  • A UV sterilizer is the single most reliable and fastest fix
  • A 3-4 day total blackout works as an alternative if you don't have a UV sterilizer
  • Water changes alone won't fix it -- they only dilute the bloom temporarily

What Is Green Water?

Green water, sometimes called 'pea soup', is caused by a bloom of free-floating, single-celled algae (commonly Chlorella-type species) suspended throughout the entire water column rather than growing on surfaces. It can turn a tank from crystal clear to opaque green within a day or two, and unlike algae on glass or plants, it can't be wiped away -- it's genuinely floating in the water itself.

Note that if your aquarium water is milky white or grey rather than green, you are likely dealing with a bacterial bloom or suspended particles. For general diagnosis of non-green turbidity, see our cloudy water guide.

Why It Grows

The most common trigger is direct or strong indirect sunlight hitting the tank, combined with available nutrients from fish waste or uneaten food. Because the algae is free-floating, it can out-compete rooted plants for light and nutrients extremely quickly once a bloom starts, which is why it tends to appear suddenly rather than build up gradually.

Step-by-Step Removal

  1. Install a UV sterilizer if you have access to one -- water passing through the unit kills the free-floating algae cells, and most tanks clear within 2-4 days of continuous use. This is the single most reliable fix.
  2. Alternatively, do a full 3-4 day blackout -- cover the tank completely to block all light. Free-floating algae generally can't survive extended darkness the way rooted plants can.
  3. Run a diatom filter temporarily if available -- it mechanically polishes the water and physically removes the suspended algae cells, giving faster visible results than either method alone.
  4. Avoid relying on water changes alone -- they dilute the bloom temporarily but don't address the light/nutrient cause, so it typically returns within days.

Prevention

Keep the tank out of direct sunlight and away from windows where afternoon sun can hit it. A permanently installed UV sterilizer is the most effective long-term prevention for tanks that are prone to recurring blooms. Fast-growing floating plants like duckweed or water lettuce can also help by out-competing free-floating algae for light and nutrients.

A Note on Algae-Eaters

No fish or invertebrate reliably controls green water -- it's a water-column problem, not a surface-grazing one. Daphnia will consume some of it but won't clear a full bloom on their own.

Frequently Asked Questions

Will a water change fix green water?
Only temporarily -- it dilutes the current bloom but doesn't address the light and nutrient conditions causing it, so the water typically turns green again within a few days.
How fast does a UV sterilizer clear green water?
Most tanks visibly clear within 2-4 days of continuous UV sterilizer use, since it kills the free-floating algae cells as water passes through the unit.
Is green water harmful to my fish?
It's generally not directly harmful to fish, though it can make it hard to see them and may indicate an underlying nutrient or light imbalance worth addressing.
Can direct sunlight alone cause a green water bloom?
Yes -- direct or strong indirect sunlight is one of the most common single triggers, which is why tanks placed near windows are especially prone to sudden green water blooms.
Do floating plants help prevent green water?
Yes, fast-growing floating plants like duckweed or water lettuce compete with free-floating algae for light and nutrients, which can help prevent blooms from establishing in the first place.