Cloudy Aquarium Water: White, Green or Brown and How to Clear It

Few things are as discouraging as waking up to a milky or green aquarium. The good news is that cloudy water is almost never an emergency in itself: it is a symptom, and its color says a lot about the cause. The bad news is that the typical reaction, draining half the tank or scrubbing the filter, usually makes the problem last longer. In this guide you will learn to identify the type of cloudiness, what causes it and how to clear the water without destabilizing the tank.

Types of cloudy water by color

The color is your main clue. This table sums up the four most common cases:

Color	What it is	When it shows up	What to do
White or milky	A boom of free-floating bacteria (bacterial bloom)	New tanks, after deep-cleaning the filter, or from overfeeding	Wait, feed very little, leave the filter alone; usually clears within days to a couple of weeks
Green	Free-floating microalgae (green water)	Too much light, direct sun, high nitrate or phosphate	Cut light, a 3-4 day blackout, UV sterilizer, regular water changes
Brown or yellowish but clear	Tannins released by driftwood, leaves or peat	After adding new driftwood or Indian almond leaves	Harmless; if it bothers you, activated carbon and water changes
Gray or full of particles	Substrate dust or fine debris	Right after filling the tank or stirring up the gravel	Let the filter work; add fine filter floss

White water: the bacterial bloom

This is the most common type of cloudiness and the most alarming. The water turns whitish, as if someone had added a drop of milk. It is caused by a rapid explosion of heterotrophic bacteria that float in the water column and feed on dissolved organic matter. It shows up mostly in newly set-up tanks during cycling, but also in established tanks after an aggressive filter cleaning, overfeeding or something rotting (a hidden dead fish, built-up food, decaying leaves).

In a new tank, a bacterial bloom is part of the maturing process and disappears on its own once bacterial colonies settle in the filter and substrate. What works:

• Patience: it normally clears within a few days, sometimes two or three weeks.
• Feed lightly: only what they eat in 2-3 minutes, or skip a day if there are fish.
• Do not touch the filter or replace its media: that is where the beneficial bacteria are settling.
• Look for leftovers: in an established tank, check behind decor for a dead animal or rotting food.
• Test ammonia and nitrite with a water test kit: if they rise, change 25-30% with conditioner.

Green water: free-floating algae

If the water has a green tint, like pea soup, it is not bacteria but single-celled algae drifting freely. Water changes alone barely help, because the algae reproduce within days. The causes are the same as for other algae: too many hours of light, direct sun on the tank and available nutrients (nitrate, phosphate or leftover ammonia in new tanks).

• Cut the light to 6 hours or less and move the tank out of direct sun.
• Do a full 3-4 day blackout by covering the tank with a blanket, with a water change before and after. Fish and plants handle it well.
• A UV sterilizer is the most effective and fastest fix: it kills the algae as they pass the lamp.
• Fix the cause long term: more plants, less food and regular water changes.

For other algae (on glass, plants or decor), see our guide to aquarium algae.

Brown water and gray water

Brown or yellowish: tannins

If the water is clear but tea-colored, the cause is almost certainly tannins leaching from new driftwood, mangrove roots, Indian almond leaves or peat. It is not dirt and it is not toxic: in fact, many blackwater species, such as the cardinal tetra, live naturally in water like this and benefit from it. It lowers the pH slightly.

If you do not like the look, you have three options: boil or soak driftwood for several weeks before adding it, run activated carbon in the filter (replacing it every few weeks) and keep up your regular water changes. Over time the wood releases less and less color.

Gray or full of particles: substrate dust

Gray water with visible particles usually appears right after filling the tank, from poorly rinsed gravel or sand dust, or after stirring it up while planting or vacuuming. It normally settles within a day or two. To speed things up, add a layer of fine filter floss to the filter and rinse or replace it when it clogs. Bottom-dwellers that sift the substrate, such as panda corydoras, kick up a few particles every day, but that is normal and should not be confused with persistent cloudiness.

Filtration: your ally against cloudy water

A properly sized, well-maintained filter prevents most cloudy-water problems. The recommended flow is 3-5 times the tank volume per hour (a 20-gallon tank needs roughly 60-100 gallons per hour), with three kinds of filter media: mechanical (sponges and floss to trap particles), biological (ceramic rings or porous balls where bacteria live) and, optionally, chemical (activated carbon for tannins and odors).

The key to maintenance is not to overdo it: rinse sponges in tank water when the flow drops, never under the tap, and do not replace all the media at once. Bottled bacteria can help shorten a bloom in new tanks or after a mishap, although they are no substitute for patience.

Common mistakes

• Changing most of the water in a new tank every time it turns white.
• Rinsing the filter under tap water or replacing all the media in one go.
• Feeding the same amount during a bacterial bloom.
• Using chemical clarifiers without finding the cause: they clump particles together, but the problem returns.
• Not rinsing the substrate before adding it to the tank.
• Leaving the tank in the sun or the lights on 10-12 hours when the water turns green.
• Mistaking tannins for dirt and doing unnecessary massive water changes.

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