Green aquarium water: full guide
Overnight, your aquarium water has turned an opaque green, “pea soup” style. Don’t panic: it isn’t dirty in the way you might think, it’s alive. An explosion of microscopic algae. Here’s why, and how to get crystal-clear water back.
The essentials. Green water is a bloom of free-floating microscopic algae, triggered by too much light (duration, intensity or direct sun) + too many nutrients (overfeeding). Fixes, from gentlest to most drastic: cut the light (even a 3–4 day blackout), limit food, water changes, and if needed a UV steriliser (the fastest and most reliable). A standard filter isn’t enough.
Where green water comes from
Unlike the algae that coat the glass, green water is made of free-floating algae (phytoplankton) that multiply throughout the whole water column. Two ingredients trigger it, and you need both:
- Excess light: lighting on too long/bright, or above all direct sun on the tank.
- Excess nutrients: nitrate and phosphate from overfeeding, a lightly planted tank or insufficient maintenance.
It’s direct sun that’s the big trigger: a tank near a window turns green far more easily.
If your tank is exposed to direct light from a window, that's almost always where the cause is. Before any treatment, move the tank or block out the window: without that fix, the water will go green again whatever you do.
The fixes, from gentlest to most drastic
1. Cut the cause (always first)
- Cut the lighting to 6–7 hrs/day, move it away from direct sun.
- Limit food (only what’s eaten in 1–2 min) → fewer nutrients.
- Regular water changes of 20–30% to export nitrate and phosphate.
2. The blackout
Very effective and free: deprive the tank of all light for 3 to 4 days (lights off, tank covered with a sheet, blinds closed). The free-floating algae die; higher plants tolerate this short fast. You then do a water change.
3. Plants (prevention)
A well-planted tank uses up the nutrients before the algae can. Fast-growing plants are a natural long-term defence against green water.
4. The UV steriliser (the fastest)
A UV steriliser passes the water in front of a UV lamp that destroys the free-floating algae: the water clears within a few days, harmlessly to fish. It’s the most reliable fix for stubborn or recurring cases:
UV steriliser
Our pick- Destroys free-floating algae: clear water within a few days
- Harmless to fish
- The most reliable fix for cases that keep coming back
Amazon affiliate link. Price and availability may change at any time.
To fix the cause for good, it’s worth tracking the nitrate and phosphate that drive the bloom. A dedicated test lets you do that:
Nitrate / phosphate test
Our pick- Measures the nutrients (nitrate, phosphate) that feed the algae
- Helps you dial in water changes and feeding
- Tackles the cause, where UV only treats the symptom
Amazon affiliate link. Price and availability may change at any time.
Before you wrap up, check you’ve covered every step:
- I've moved the tank away from direct sun
- I've cut the lighting to 6-7 hrs/day
- I'm limiting food and doing water changes
- I'm considering a 3-4 day blackout
- I have plants consuming the nutrients (prevention)
- For stubborn cases: a UV steriliser
My approach: fixing the cause + a blackout sorts out most cases for free. I save the UV steriliser for tanks where green water keeps coming back (often the ones you can't move away from the light). UV treats the symptom, not the cause — keep that in mind.
Conclusion
Green water isn’t dangerous but it’s no fun to look at: it signals too much light and too many nutrients. Cut the cause first (sun, photoperiod, food), try a blackout, plant to prevent it, and bring out the UV steriliser for the stubborn cases. Crystal-clear water is within reach.
For white or cloudy water (a different problem), see “Cloudy aquarium water: full guide”. And for algae on surfaces, “How to get rid of green hair algae”.
Frequently asked questions
Why is my aquarium water turning green?
Green water is an explosion of free-floating microscopic algae (phytoplankton), caused by excess light (duration, intensity or direct sun) combined with excess nutrients (nitrate, phosphate from overfeeding). The two together trigger the bloom.
Is green water dangerous for fish?
Not directly: fish generally cope well with green water, and some even thrive in it. The main downside is the look and, in a heavy bloom, a drop in oxygen at night. Best to treat it.
How do I get rid of green water quickly?
The fastest, most reliable method is a UV steriliser, which clears free-floating algae within a few days. Without UV: cut the light right back (even a few-day blackout), limit food and do water changes.
Does a blackout work against green water?
Yes. Depriving the tank of all light for 3 to 4 days (tank covered, lights off) kills the free-floating algae. Higher plants tolerate this short light fast. You then combine it with water changes and a corrected cause.
Does a filter remove green water?
A standard filter doesn't trap microscopic algae (too fine). Only a UV steriliser or a very fine filter (as a complement) deals with it. The real fix is still to cut the cause: too much light and too many nutrients.
