Guide
Green water (algae bloom) fix
Free-floating algae: reduce light and nutrients, water changes.
- What it is: Free-floating single-celled algae (Chlorella and relatives) suspended throughout the water column - not a film on the glass, not surface scum. Your water turns opaque green like pea soup.
- Root cause: Too much light combined with elevated nutrients (nitrate above ~20 ppm, phosphate above ~0.5 ppm). Both drivers need to be present - light alone or nutrients alone rarely cause a full bloom.
- Fastest fix: A 3-4 day blackout (cover the tank completely) while cutting feeding to zero. This crashes the bloom without chemicals.
- Most reliable fix: A UV steriliser. Water passing through the UV chamber is exposed to light that ruptures algae cells. Clears green water in 24-72 hours and keeps it from returning as long as it runs.
- Prevention: A timer-controlled photoperiod of 6-8 hours, weekly nitrate testing, and not overstocking or overfeeding. Green water is almost always a symptom of an unbalanced tank, not bad luck.
- Fish safety: Mild blooms are harmless. Dense blooms can crash dissolved oxygen at night as algae respire - watch for fish gasping at the surface and act fast if you see it.
What green water actually is
Green water is caused by a population explosion of microscopic single-celled algae - primarily Chlorella species and other green microalgae - suspended freely in the water. Because the cells are measured in microns, no mechanical filter with standard media can catch them all. They multiply exponentially once conditions are right, which is why a tank can go from clear to pea-soup green within two or three days.
It looks dramatic, but it's one of the more solvable algae problems in the hobby. Unlike black beard algae or staghorn, which anchor to surfaces and require targeted chemical or manual removal, green water responds quickly to changes in light and nutrients because the algae have no physical refuge. Take away what they need, and the bloom collapses.
The key distinction to make before you start treating: is the water genuinely green and opaque, or is the glass tinted with a green biofilm? If you can wipe it off the glass with a cloth, you're dealing with green spot or green dust algae - a different problem. True green water clouds the water itself. Shine a torch through the tank at night and you'll see the light scatter through the suspended cells.
Why your tank turned green
Green water needs two things simultaneously: light and dissolved nutrients. Remove either one and the bloom cannot sustain itself. Most guides stop at "too much light," but that's incomplete advice. Tanks sitting under strong lighting for years can stay crystal clear if nitrate and phosphate are kept low. Equally, a heavily fed, overstocked tank in a dim corner can bloom if it gets a week of direct sunlight through a window.
Light
The most common culprits are photoperiods over 10 hours and tanks positioned where they receive direct or indirect sunlight during parts of the day. A tank light running on a timer for 8 hours can still bloom if morning sun hits it for 4 hours before the timer kicks in - giving the algae an effective 12-hour day you didn't plan for.
Light intensity matters too. High-output planted tank lights running at full intensity in a low-tech setup without enough plant mass to consume the available light and CO₂ will almost always feed an algae problem.
Nutrients
Nitrate above 20-30 ppm and phosphate above 0.5-1.0 ppm are the usual suspects. These build up through overfeeding, overstocking, infrequent water changes, and decaying plant matter sitting on the substrate. See our guide on aquarium water parameters for target ranges across the full parameter set.
New tanks are particularly vulnerable. During and just after cycling, nutrients are in flux and plant life hasn't established yet - there's nothing competing with the algae. If you're not sure your tank is properly cycled, read how to cycle a fish tank before chasing the algae problem.
Doing a single large water change and calling it done. One 50% water change halves your nitrate - if it was at 40 ppm, you're now at 20 ppm. That's still enough to fuel a bloom. You need to change the conditions, not just dilute them once.
Overstocking and overfeeding
More fish means more waste, which means faster nutrient accumulation. A tank stocked beyond its realistic capacity will continuously push nitrate and phosphate upward no matter how often you change water. Use our free stocking calculator to check whether your fish load is realistic for your tank volume. If you're consistently battling nutrient problems, stocking density is always worth reviewing.
Overfeeding is the single easiest habit to fix. Feed only what fish consume in 2 minutes, once or twice a day. Uneaten food starts decomposing within hours and releases ammonia directly into the water.
How to clear green water
Option 1: The blackout
Cover the tank completely - sides, top, front - with cardboard or towels so zero light reaches the water. Leave it for 3 to 4 days. Don't feed during this period. Don't peek. Most free-floating algae will die off without light, and because the cells are already suspended in the water column, the dead matter gets picked up by your filter.
After the blackout, do a 30-40% water change, clean your filter media in old tank water (not tap water - you'll kill the beneficial bacteria), and resume a controlled photoperiod of 6-8 hours using a timer. Resume feeding at reduced levels.
The blackout works reliably if you actually commit to it. Four days feels like a long time, but your fish will be fine without food for that period. Healthy adult fish can fast for 7-10 days without harm.
If you have live plants, they'll tolerate 3-4 days of darkness better than the algae will - established plants have energy reserves in their roots and rhizomes. Fast-growing stem plants may melt slightly but will recover. Mosses and low-light species handle it without issue.
Option 2: UV steriliser
A UV steriliser is the most effective tool for green water, and the hobby broadly agrees on this. Water is pumped slowly past a UV-C lamp inside a sealed chamber. The ultraviolet light damages the DNA of single-celled organisms - including free-floating algae - killing them before they return to the tank. Dead cells clump together (a process called flocculation) and get caught by your mechanical filter.
Flow rate matters. The UV unit needs to be sized for your tank volume, and the water must move slowly enough through the chamber for the UV dose to be effective. Running too high a flow rate is a common reason UV sterilisers underperform. Check the manufacturer's stated flow rate for algae control (usually lower than their maximum rating) and match it.
Most tanks see clear water within 24-72 hours. The steriliser can run continuously as a preventive measure after that, though it's not essential if you've fixed the underlying light and nutrient problem.
UV sterilisers are sold as premium items but mid-range units from reputable brands work perfectly well for green water in home aquaria. You don't need to spend a lot. What matters is correct sizing and flow rate, not the price tag.
Option 3: Diatom filters and polishing pads
Diatom filters use diatomaceous earth as a filtration medium and can mechanically remove algae cells from the water because the pore size is small enough to catch them. They're effective but designed for periodic use, not continuous running. A polishing pad added to your existing filter will catch some dead algae cells after a blackout or UV treatment, helping the water clear faster - but it won't fix a live bloom on its own.
Water changes during a bloom
Water changes alone won't clear green water quickly - you'd need to replace almost all the water to make a meaningful dent, and you'd be stressing the fish. But they're essential for reducing the nutrients that sustain the bloom. Do 25-30% changes every 2-3 days while you're treating, and keep testing. For guidance on frequency, see how often to do water changes.
The photoperiod most freshwater community tanks need. More than this without significant plant mass is feeding algae, not your fish or plants.
Plants as long-term competition
Fast-growing plants compete directly with algae for the same nitrate, phosphate, and CO₂. A well-planted tank with species like Vallisneria, Egeria densa, hornwort, or fast stem plants like Rotala rotundifolia will naturally suppress algae blooms by stripping nutrients from the water before algae can use them.
The mistake here is adding fertiliser and CO₂ without enough plant mass to consume both. In a lightly planted tank, fertiliser directly supplements algae. Dose nutrients only to meet plant demand - if you're seeing algae, you're almost certainly overdosing relative to your plant load.
You don't need a high-tech CO₂-injected setup for this. A low-tech tank with fast-growing, undemanding plants under 6-8 hours of moderate light, with sensible stocking and regular water changes, is one of the most algae-resistant setups in the hobby.
What to fix permanently
Clearing the current bloom is the easy part. The harder part is making sure it doesn't come back in three weeks. These are the habits that prevent recurrence:
- Use a timer. Consistent photoperiod on a mechanical or smart timer. No guessing, no forgetting to switch the light off.
- Test regularly. Nitrate monthly at minimum; weekly if you've had algae problems. Phosphate is worth testing once to establish a baseline. App-aquatic lets you log test results over time so you can spot trends before they become problems.
- Match stocking to the tank. More fish than your filtration and water changes can handle means chronic nutrient excess. The nitrogen cycle explains why this matters mechanically.
- Feed less than you think you need to. Every experienced fishkeeper who revisits their feeding habits reduces the amount. Fish in aquaria almost always have enough food before you think they do.
- Deal with windows. If your tank gets any direct sunlight, move it or fit a blind. This is non-negotiable - sunlight is far more intense than any aquarium light, and you cannot fight a bloom that's being powered by sun.
A dense green water bloom can crash dissolved oxygen levels at night. During the day, algae photosynthesize and produce oxygen. After lights out, they consume oxygen through respiration - and with millions of cells in the water, that draw-down can be significant. If fish are hanging at the surface or gasping first thing in the morning, increase surface agitation immediately and treat the bloom as urgent.
Track your parameters and get care reminders with App-aquatic.
Get the free appIs green water dangerous for fish?
Mild blooms are generally harmless - many fishkeepers actually use green water deliberately to raise fry, since the microalgae are nutritious. Dense blooms are a different story. The overnight oxygen crash from heavy algae respiration can stress or kill fish. If the water is so green you can barely see your fish, treat it urgently and increase surface agitation until it clears.
How long does a blackout take to work?
Three to four days of complete darkness kills most free-floating algae. The water may still look cloudy with dead cells afterward - give it another day or two with your filter running and do a water change. Don't cut the blackout short at two days because it seems to be working; incomplete blackouts often see the bloom rebound once light returns.
Will green water clear on its own?
Occasionally, yes - if conditions shift (a few cloudy weeks, reduced feeding during a holiday) the bloom can retreat. But if you don't fix the underlying light and nutrient levels, it will come back. Waiting it out without changing anything is not a strategy.
Can I use algaecides or chemical treatments?
Most algaecides work by killing plant cells indiscriminately, which means they'll damage or kill your live plants and can harm invertebrates and sensitive fish. They also do nothing to fix the root cause, so the bloom returns once the chemical wears off. Stick to the blackout or UV approach - they're more effective and don't create collateral damage.
My tank is new and turned green after two weeks - what's happening?
New tank syndrome. Your biological filtration isn't established yet, nutrients are uncontrolled, and there's no plant competition in place. A new tank bloom is extremely common. Reduce your photoperiod to 6 hours, cut feeding right back, and do 25% water changes every 2-3 days. Read the guide on how to cycle a fish tank - sorting out the cycle is the real fix here, and the algae will settle down once the tank matures.
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