Cloudy aquarium water, by color
Four different problems share one symptom. Identify which one you have before you buy anything, because for three of them the correct product is patience.
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Identify the color first
Cloudy water is a symptom with four common causes, and they need opposite responses. Buying a clarifier before you know which one you have is how people end up treating a bacterial bloom that would have cleared itself in a week.
| Looks like | Usual cause | What fixes it |
|---|---|---|
| White or milky haze | Bacterial bloom, usually in a new or recently disturbed tank | Time. Do not treat it. |
| Green water | Free-floating algae - too much light and nutrient | Shorten the photoperiod; block daylight |
| Brown or tea-colored | Tannins from driftwood or botanicals | Nothing, or carbon if you dislike it |
| Gray, right after setup | Substrate dust | Mechanical filtration and a day |
Note how few of those rows want a product. That is the honest shape of this problem, and it is why this page is mostly about diagnosis.
White or milky: a bacterial bloom
This is the common one, and it is the one people panic about. A cloud of free-floating heterotrophic bacteria multiplies in response to a sudden supply of dissolved organics - a new tank, a big feed, a dead snail, a filter clean that released trapped detritus.
It looks alarming and it is usually harmless. The bloom consumes the organics and then crashes, typically within a few days to a couple of weeks. The colony cannot sustain itself once the food is gone.
What to do: test ammonia and nitrite, feed nothing for a day or two, and wait. If the tank is new and not yet cycled, this is a normal stage of the cycle and the cloud will clear as the nitrifying bacteria establish on the media rather than in the water.
What not to do: a large water change removes the food and the bacteria in proportion, so it dents the cloud without ending it - and in an uncycled tank it slows the cycle. Do not clean the filter, which is where you want bacteria living. And do not dose a clarifier, which clumps particles for the filter to catch without addressing why they are there.
The one reading that changes the advice
If there is a fish in the tank and ammonia or nitrite reads above zero, the cloudiness is the least of it - that is an uncycled or crashed tank and the water change is no longer optional. A liquid test kit is the only thing that distinguishes “harmless bloom, wait it out” from “the fish is in trouble.” The two look identical through the glass, which is the entire argument for owning one.
Green: free-floating algae
Green water is algae suspended in the water column rather than growing on surfaces. The cause is always the same pair: light and nutrients, in excess of what the plants are using.
In a nano tank the usual culprits are a fixture running ten or twelve hours a day, direct sunlight from a nearby window, or overfeeding. The fix is the same order every time:
- Shorten the photoperiod to six to eight hours on a timer - why that number is on the lighting page.
- Move the tank out of direct sun, or block the window side. Sun delivers far more energy than any nano fixture and you do not control it.
- Feed less, and vacuum the substrate at the next change.
- Add fast-growing plants to compete for the same nutrients - floating plants are the fastest option available to a low-tech tank.
A blackout - three days with the light off and the tank covered - clears green water reliably and does nothing about the cause. Use it to reset, then fix the light and the feeding, or it returns.
Brown or tea-colored: tannins
New driftwood, catappa leaves and other botanicals leach tannins, which stain the water amber to tea-brown. This is not cloudiness in the suspended-particle sense - the water stays clear, it just has a color.
It is harmless, it softens the water slightly, and a great many keepers prefer it for a betta specifically. If you do not, activated carbon in the filter removes it within days - the media page covers why we otherwise suggest leaving carbon out and using the basket space for biological media. Tannins are one of the two genuine reasons to run it.
Wood leaches most heavily when new and tapers over months. Soaking or boiling it before it goes in shortens that.
Gray and gritty, right after setup
Fine dust from substrate, especially a powder-grade soil. It is mechanical, not biological, and it settles or gets caught by the filter within a day or two.
The way to avoid it entirely is at fill time: never pour water straight onto the bed. Onto a saucer, a bag laid over the substrate, or the back of your hand - the substrate comparison covers why the finer soils need this most. And do not rinse an aquasoil, which washes out the nutrients and turns it to mud.
If it persists past a couple of days, a fine mechanical pad - floss or fine foam - in the filter will polish it out. That is the one product-shaped answer on this page, and it costs very little.
What we would not buy
Water clarifiers. They flocculate suspended particles so the filter can catch them. On substrate dust that is redundant, because the filter catches it anyway in a day. On a bacterial bloom it treats the symptom while the cause continues. On green water it does nothing about light. We have not found a case in a nano tank where it is the right first purchase.
UV sterilizers. Genuinely effective against green water, and priced for tanks many times this size. In 8.5 gallons, a timer and a curtain do the same job.
A full water change and substrate rinse. The nuclear option people reach for on day three of a bloom. It removes the bacteria you are trying to grow, restarts the cycle, and the cloud usually returns.
The diagnostic, in order
- 01
Look at the color in daylight
White, green, brown or gray. Tank lighting distorts this - hold a white card behind the glass and look at it by a window.
- 02
Test ammonia and nitrite
This is the step that decides whether you have a cosmetic problem or a livestock problem. Everything else is secondary to that reading.
- 03
Ask what changed in the last week
New tank, new wood, a big feed, a filter clean, a dead snail, the clocks changing your light schedule. The cause is almost always a recent event.
- 04
Apply the fix for that color - and wait
Three of the four causes resolve on their own once the input stops. Give any change a week before concluding it did not work.
What we cannot tell you
Whether a given cloud is harmful to your fish, beyond what a test kit reads. We do not diagnose illness anywhere on this site, and cloudy water is a water symptom rather than a fish one - if the water tests clean and the fish is behaving oddly, the cloudiness is probably not the story.
What we would buy for this
Chosen from the same registry as our roundups, on published specifications and the arithmetic they imply - not on testing we have not done.
API Freshwater Master Test Kit
The liquid test kit the whole hobby ends up owning, and the only product on this site that can actually tell you whether your tank is cycled. It reads pH, high-range pH, ammonia, nitrite and nitrate, and API publishes over 800 tests per kit - which in a nano tank you will be years getting through. The nitrate test is the one people get wrong: bottle two needs banging on a hard surface for a full 30 seconds or it reads low.
Best for: Anyone cycling a tank, which is anyone setting one up
Skip it if: You want a two-second answer. This is five test tubes, a dropper count and a five-minute wait, and strips are the product for that.
Fluval AquaClear 20 Power Filter
The hang-on-back that everyone eventually buys, rated 5-20 gallons. Its advantage is the media basket: you choose what goes in it, and you can pack it with foam and BIOMAX and skip the carbon entirely. Its problem in a nano is that a 5-gallon sits at the very bottom of its range, so you will be running the flow control near minimum and probably baffling the waterfall as well.
Best for: A 10-gallon where you want media capacity, not a cartridge
Skip it if: The tank is 3 gallons. The bottom of AquaClear's published range is 5 gallons and there is no honest way to read that as covering a 3.
Seachem Prime
The dechlorinator whose published dosing makes it the obvious nano choice: 5 mL treats 50 US gallons, so a 5-gallon water change needs half a milliliter. A 100 mL bottle is therefore roughly two hundred water changes. It also binds ammonia and nitrite, which is the property that turns a cycling accident into a survivable week.
Best for: Every water change, and every cycling emergency
Skip it if: You need something you can dose by the capful into a nano. At this concentration a capful in a 5-gallon is a ten-times overdose - you are measuring in drops.
Frequently asked
Why is my fish tank cloudy after a water change?
Usually a bacterial bloom triggered by disturbed detritus, or fine substrate dust put back into suspension. Both clear on their own - test ammonia and nitrite to confirm nothing else is happening, then wait.
How long does a bacterial bloom last?
Typically a few days to a couple of weeks. The bloom consumes the dissolved organics that caused it and then crashes. Large water changes and filter cleans prolong it rather than shortening it.
Should I use a water clarifier?
In a nano tank we would not. It clumps particles for the filter to catch without addressing the cause - which for three of the four kinds of cloudiness resolves on its own within days.
Why is my aquarium water green?
Free-floating algae, driven by too much light for too long plus available nutrients. Shorten the photoperiod to six to eight hours, get the tank out of direct sunlight, and feed less. A three-day blackout clears it but does not fix the cause.
Is brown water bad for a betta?
Tannins from driftwood or botanicals stain the water and are harmless - many keepers prefer them for a betta. If you dislike the color, activated carbon removes it in a few days.
Cloudy water in a brand-new tank - is that normal?
Yes, and it is expected. A new tank blooms while the biological filter establishes. The important thing is that there should be no fish in it yet - cycle first, and let the cloud come and go on its own.