Guide
Why is my aquarium water cloudy?
White, grey, or green cloudiness can mean different things. How to tell and what to do.
- White or milky water in a new tank: almost certainly a bacterial bloom - part of the nitrogen cycle. Leave the filter alone and it will clear on its own within a few days to two weeks.
- White or grey water in an established tank: stirred substrate, a dying filter colony, or a sudden ammonia spike. Test your water before doing anything else.
- Green water: free-floating single-celled algae (Euglena or similar). Caused by too much light, excess nutrients, or both. Won't clear without addressing the cause.
- Never blast your filter media when water is cloudy - you'll destroy the beneficial bacteria and make things worse.
- Flocculants and chemical clarifiers treat the symptom, not the cause. Use them only once you know what's actually wrong.
What cloudy water is actually telling you
Cloudy water is a symptom, not a problem in itself. The mistake most fishkeepers make is reaching for a bottle of clarifier the moment visibility drops. The colour and context of the cloudiness tell you almost everything you need to know - and the fix is completely different depending on which type you're dealing with.
There are three distinct types: white/milky, grey, and green. Each has a different cause, a different timeline, and a different response. Getting them confused wastes time and can actively harm your tank.
White or milky cloudy water
Bacterial bloom in a new tank
If your tank is less than six weeks old and the water turns milky white within the first week or two of adding fish, this is almost certainly a bacterial bloom. You've introduced an ammonia source (fish waste, food) before the biological filter has enough bacteria to process it. The free-floating bacteria suspended in the water column are the cloudiness itself - they reproduce explosively until the cycle catches up.
This is normal. It doesn't mean your tank is dirty or that you've done something wrong. The nitrogen cycle takes time to establish, and a bloom is a visible sign that the process is underway.
During a bacterial bloom, keep feeding to an absolute minimum - once every other day is fine for most fish. Less food means less ammonia, which means the bloom clears faster. Your fish won't starve.
What you should do: nothing dramatic. Keep the filter running, maintain normal water temperature, and do small water changes (20-25%) every two to three days if ammonia climbs above 1.0 mg/L. The bloom typically clears within 2-10 days once the cycle stabilises. Read the full guide on how to cycle a fish tank if you're not sure where your tank is in the process.
Cleaning or rinsing filter media during a bacterial bloom. Your filter sponge and ceramic media are exactly where you want those bacteria to colonise. Washing them in tap water - even rinsing them under a tap briefly - kills chlorine-sensitive bacteria and restarts your cycle. If you need to rinse filter media, always use water taken from the tank itself.
White cloudiness in an established tank
If your tank has been running for months and suddenly goes white, a bacterial bloom is still possible - but now it's a warning sign rather than a normal phase. Common triggers include:
- Overstocking or sudden increase in bioload - adding too many fish at once overwhelms the filter.
- Dead fish or uneaten food decomposing - a spike in ammonia kicks off a secondary bloom.
- Medicating the tank - some antibacterial treatments wipe out filter bacteria as collateral damage.
- New filter media without seeding - replacing all your media at once gives you an uncycled tank again.
Test for ammonia and nitrite immediately. If either is above 0 mg/L in an established tank, you have a problem that needs addressing. Check your aquarium water parameters guide for safe ranges by species.
Even 0.25 mg/L causes gill damage over time. If your established tank is cloudy, test ammonia first - it's the fastest way to rule out a cycle crash.
Grey cloudy water
Grey cloudiness is almost always physical - tiny particles suspended in the water rather than living organisms. The most common cause in new setups is substrate dust. Gravel, sand, and especially fine-grain substrates like Fluval Stratum or ADA Aqua Soil release fine particles when first submerged, and these can take several days to settle even with good filtration.
In established tanks, grey water usually means something has been disturbed: a gravel clean that stirred up waste, a new decoration, or substrate that's been disrupted by digging fish. It's not harmful, but it signals your filter is struggling to keep up.
When setting up a new tank with sand or fine substrate, pour water in slowly over a plate or your hand to minimise disturbance. Even so, plan for 24-72 hours of grey water before it clears. A fine filter floss pad in the last stage of your filter speeds this up significantly.
For persistent grey water in an established tank, check that your filter flow rate is adequate for your tank volume - a general rule is turnover of 4-6× the tank volume per hour for a community tank. Upgrading to finer mechanical filtration media (filter floss or a fine sponge) will catch the particles your current media is missing.
Green cloudy water
Green water is a completely different problem. It's caused by a bloom of free-floating microscopic algae - most often Euglena or related phytoplankton - suspended throughout the water column. Unlike the algae that grows on glass or decorations, this stuff won't respond to scrubbing. You need to fix the conditions driving it.
The two primary causes are excess light and excess nutrients. Usually it's both. Common scenarios:
- Tank positioned near a window with direct or strong indirect sunlight
- Lights left on for 10-14 hours a day (8 hours is the standard recommendation for most planted and community tanks)
- High nitrates from infrequent water changes or overstocking
- Overfeeding driving up phosphate levels
A UV steriliser will clear green water - but it won't fix the underlying problem. If your light schedule and nitrate levels don't change, the algae bloom will return within weeks of turning the steriliser off. Sort the cause first, then use UV as a finishing tool if needed.
How to clear green water
Start with a 3-day blackout if your tank has fish that can handle it (most can). Cover the tank completely - no light whatsoever. The algae will die off faster than your plants lose meaningful energy reserves. After the blackout, set your light timer to a strict 7-8 hour photoperiod.
Alongside the blackout, do a 30-40% water change to dilute the nutrient load, cut feeding to once daily with no leftovers, and check your nitrate level. Ideally nitrates should sit below 20 mg/L for a community tank. If they're regularly spiking to 40 mg/L or above, revisit how often you're doing water changes.
If the tank is near a window, reposition it or add a blackout blind on that side. Ambient daylight is the single most overlooked cause of persistent algae blooms - your LED timer means nothing if the tank gets six hours of afternoon sun on top of it.
Don't do a blackout if you have a heavily planted tank that relies on CO2 injection and high light - you'll cause more plant melt than algae die-off. In planted high-tech setups, address the nutrient imbalance and reduce the photoperiod gradually instead.
Chemical clarifiers: when they help and when they don't
Flocculants like Seachem Clarity or API Accu-Clear work by clumping fine particles together so your mechanical filtration can catch them. They're genuinely useful for grey water caused by substrate dust, and can help speed up the tail end of a bacterial bloom once the cycle has stabilised.
They do not work on green water algae - the particles are too small. And they will do nothing for an ongoing bacterial bloom if ammonia is still elevated, because new bacteria keep replacing the ones you've clumped out.
Most people over-dose clarifiers. One treatment at the recommended dose is enough. Overdosing can cause oxygen depletion as the clumped organic matter decays in the filter, which stresses fish. If one dose hasn't helped within 24 hours, the problem is not physical debris and clarifier is the wrong tool.
What most guides miss
Temperature affects how fast cloudiness clears
Bacterial blooms clear faster in warmer water because bacterial metabolism speeds up. If your tank is sitting at 22°C (72°F) during a bloom, nudging it up to 25-26°C (77-79°F) shortens the cycle duration. Don't go above 28°C (82°F) unless you're keeping tropical species that require it.
Your stocking level matters more than you think
Recurrent cloudiness - white water that keeps coming back in an established tank - is often a sign the tank is overstocked for its filtration capacity. More fish means more ammonia, which means the biological filter is perpetually struggling to keep up. Use the free stocking calculator to check whether your bioload is realistic for your setup before changing anything else.
Seeding the filter accelerates recovery
If you're cycling a new tank or recovering from a crashed cycle, adding a pinch of media from a healthy established tank - or using a commercial bacterial starter like Tetra SafeStart or Dr. Tim's One and Only - cuts the bloom phase significantly. It's not magic, but seeding means you start with a functioning colony rather than from scratch.
App-aquatic lets you log ammonia, nitrite, and nitrate readings over time so you can see exactly when your cycle peaks and when it stabilises. It's the clearest way to know whether that bacterial bloom is fading or getting worse.
Track your parameters and get care reminders with App-aquatic.
Get the free appHow long does a bacterial bloom last in a new tank?
Usually 2-10 days, though in tanks with heavy stocking or warm water it can stretch to two weeks. If white cloudiness persists beyond that, test your ammonia and nitrite - something is stalling the cycle.
Is cloudy water dangerous for fish?
The cloudiness itself isn't the danger - what's causing it might be. A bacterial bloom accompanied by elevated ammonia (above 0.5 mg/L) is harmful to fish. Green water algae can deplete oxygen overnight when the lights go off. Always test before deciding whether it's a cosmetic issue or a real risk.
My tank was clear for months and suddenly went cloudy. What happened?
In an established tank, sudden cloudiness usually means something has disrupted the biological filter - a dead fish you didn't notice, a medication dose, replacing too much filter media at once, or a significant overfeed. Check for dead animals first, then test ammonia and nitrite to assess the cycle's health.
Will a water change fix cloudy water?
Partially, and temporarily. A 25-30% water change dilutes ammonia and suspended particles, which helps - but if the underlying cause hasn't changed, the cloudiness returns. Water changes are supportive, not a cure on their own.
Can I add fish while the tank is cloudy?
Not during a bacterial bloom in a new tank - ammonia and nitrite levels are likely unsafe. Wait until both read 0 mg/L on a test kit and the water has cleared. If you're unsure what fish to start with once the cycle is complete, the best fish for beginners guide is a good place to start.
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