Clarity and color
The water has turned green and I cannot see through it
Green water is a bloom of single-celled algae suspended in the water column, and it needs two things at once: light and dissolved nutrients. Test nitrate before anything else, because a high reading tells you the nutrient side is yours to fix. Reducing light is the fastest lever you have, and it is the one most people reach for last.
Act this week
Test these first
Before anything goes into the tank and before anything comes out of it, get these numbers. They cost fifteen minutes and they decide everything that follows.
- Nitrate — the nitrate ion — note that drinking water standards are written as nitrate-nitrogen, which is a different number
- Ammonia — total ammonia — the ammonium ion plus free ammonia, added together
- pH and KH — acidity of the water, plus the carbonate buffer that resists changing it
What is usually behind it
| Cause | How to confirm | Time to resolve |
|---|---|---|
| Too much light, usually daylight rather than the tank light | The tank gets direct sun for part of the day, or the light is on more than about eight hours. The green is often worse on the window side of the glass. Cover the tank completely for two days and see whether it slows. | Visible improvement in five to ten days |
| High nitrate from a water change interval that has slipped | Nitrate reads above 40 ppm just before a change, or the last change was more than two weeks ago. Test tap water too — some supplies arrive with nitrate already in them, which changes what a water change can achieve. | Two to four weeks of consistent changes to shift the baseline |
| A recent ammonia event that fed the bloom | The green followed a cycle problem, a new tank, a filter clean, or a die-off. Ammonia may already have returned to zero by the time you look, which is why it gets missed. | One to three weeks after the underlying ammonia problem is resolved |
| Overstocking or overfeeding keeping nutrients permanently high | Nitrate climbs back above 40 ppm within three or four days of a large water change. The tank has more fish, or more food going in, than its volume comfortably carries. | Four to eight weeks to change the tank's baseline |
Too much light, usually daylight rather than the tank light
How to confirmThe tank gets direct sun for part of the day, or the light is on more than about eight hours. The green is often worse on the window side of the glass. Cover the tank completely for two days and see whether it slows.
What to doCut the photoperiod to six hours on a timer and move the tank or the curtain so no direct sun reaches it. Do not black the tank out entirely if there are plants you want to keep — a shorter day does the same work more slowly and less brutally.
Time to resolve: Visible improvement in five to ten days
High nitrate from a water change interval that has slipped
How to confirmNitrate reads above 40 ppm just before a change, or the last change was more than two weeks ago. Test tap water too — some supplies arrive with nitrate already in them, which changes what a water change can achieve.
What to doChange 30% now and vacuum the substrate while you do it, then hold a weekly schedule. Water changes will not clear the green on their own — the algae reproduce faster than you can dilute them — but they remove the fuel so the light reduction can finish the job.
Time to resolve: Two to four weeks of consistent changes to shift the baseline
A recent ammonia event that fed the bloom
How to confirmThe green followed a cycle problem, a new tank, a filter clean, or a die-off. Ammonia may already have returned to zero by the time you look, which is why it gets missed.
What to doDeal with the ammonia source first — that is the actual fault. The bloom is a consequence and will fade once the nitrogen supply is back to normal and the light is under control.
Time to resolve: One to three weeks after the underlying ammonia problem is resolved
Overstocking or overfeeding keeping nutrients permanently high
How to confirmNitrate climbs back above 40 ppm within three or four days of a large water change. The tank has more fish, or more food going in, than its volume comfortably carries.
What to doReduce feeding to once a day and only what is gone in a minute. Add fast-growing plants if you can — they compete for exactly the nutrients the algae are using, and unlike a water change they work continuously.
Time to resolve: Four to eight weeks to change the tank's baseline
What not to do
These are the moves that make this particular problem worse, and they are all common. Most of them come from wanting to do something substantial when the right answer is something small.
- Do not do a 90% water change hoping to dilute it away. Free-floating algae repopulate the remaining water within days and you will have disturbed everything else for nothing.
- Do not leave the light on longer because the plants look pale. Green water shades them; more light feeds the algae faster than it feeds the plants.
- Do not scrub the glass daily and call it progress. The algae are in the water, not on the surface.
- Do not combine a blackout with heavy feeding. Anything that dies during a blackout becomes the next bloom.
When this stops being a water question
Green water is not dangerous to fish in itself, but a heavy bloom consumes oxygen overnight and a tank can be noticeably short of it by dawn. If fish are at the surface in the morning, add aeration immediately and test the water. Any fish that stays unwell once the water reads correctly needs an aquatic veterinarian rather than a product.
Keeping it from coming back
The two levers are light and nitrate, and both are habits rather than purchases. A timer that holds the light to six or seven hours removes the single most common cause, and it removes it permanently rather than for a week. Keep the tank out of direct sun, feed once a day, and change water on a schedule you actually keep — the tank that never lets nitrate build past about 20 ppm rarely gets green in the first place.
Reference for this page
Equipment involved
- Test Kitsknowing the number instead of guessing it
- Water Conditioners & Additivesmaking tap water safe to put in a tank
- Filter Media & Cartridgeswhat actually does the filtering, and when it gets replaced
Related problems
Act this week
Act this week
Watch it
Published sources
- US EPA — Nutrient Pollution
- NOAA National Ocean Service — Harmful Algal Blooms
- USGS Water Science School — Dissolved Oxygen and Water
Published figures on this page come from the documents above. Where a value depends on species, pH or temperature, that is stated rather than averaged into a single number.