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Why is My Aquarium Water Cloudy?

We often receive questions about fixing cloudy water in a hobbyist’s aquarium or the water appearing to turn a different color. While not all environments have crystal-clear water and a slight tint to your tank isn’t necessarily a bad thing, water that is noticeably “tinted” or cloudy can be a symptom of an underlying issue in your aquarium. The color of the water can tend to point to a certain problem even if you haven’t yet tested the water quality. In fact, it may even be telling you what exactly you should test for next!

Why is my aquarium water green?

Planted TankThis is probably the most common and the simplest to diagnose. More often than not, green water is the sign of an algae bloom. Algae-eating fish or other critters won’t usually eat this type of algae. The algae, usually a single-celled form, is suspended in the water. The most common cause I see for these blooms in aquariums especially is high phosphate levels and this will be the first test I will always recommend. Phosphates come into the tank most often from the source water; if your water source is well water especially, phosphates may have leeched in through the soil or through nearby farms or gardens where fertilizers may have entered the groundwater. The phosphate levels may not be high enough to affect our health but in the aquarium, they can build up to levels where they are fertilizing the algae blooms and possibly causing other issues to sensitive fish and invertebrates. If you suspect this may be happening in your aquarium, grab a phosphate test kit to test both the aquarium and your source water. If the source water has phosphates, switch to a different source like RO (Reverse Osmosis) filtered water. Filter media can also help remove the phosphates already in the aquarium and regular small frequent water changes with phosphate-free water will help cut them down.

Another cause for green water may be lighting although this will affect algae on the surfaces of the aquarium as well as the water itself. If the lights on the aquarium are on for too long (over about 8-10 hours per day), this may be overfeeding the algae naturally in the water. Also, if the bulbs are older than about 6-8 months, the spectrum (“color”) of the light itself will degrade to a more yellowish color that isn’t as useful to healthy plants but will still feed the nuisance algae. Try decreasing the duration of the lights or getting new bulbs if either of those apply. If the bloom still hasn’t gotten better, test the phosphate!

Why is my aquarium water cloudy?

This is the other very common colored-water question. Usually, the water is white and milky. Whenever we hear this, the next question will always be “How long has this tank been set up with fish?” or “Have you restarted this tank lately (removed more than a third to a half of the water)?” A milky white cloudy water color to the water is a sign of a bacteria bloom which usually happens during the Nitrogen Cycle Cycling Process of a new tank or if a tank is becoming reestablished after a large water change, medication cycle or other event. This cloudiness will usually clear up on its own; try to resist the urge to do water changes since this will only make the Cycle last longer and take longer for the bacteria population that needs to grow to take care of this on its own. You can test the water during this time to make sure everything else is normal, keeping in mind that while a tank is Cycling, you may see spikes in Ammonia and Nitrite.

Why is my aquarium water yellow?

Yellowish water is usually simply dirty. This is usually a result of overcrowding or overfeeding and may also be a sign of harmfully high Ammonia and/or Nitrite levels. Test the water to see if this is the case and take a good look at the stocking levels of your tank compared to its size and filtration. If you have four goldfish in a 10 gallon aquarium, it is overcrowded and the waste they produce is polluting the water. If you have two large Oscars in a 55-gallon tank with one small power filter, it is overcrowded and underfiltered. Take a look at your feeding routine too; you may be feeding the tank more than it needs and the leftover food (or the waste the fish produce after pigging out) could be fouling up the water. To fix this cloudiness, consider getting a large tank or cutting back on the fish you have in it, invest in a larger, more powerful filter, and consider if you need to feed the fish less. A filter media with carbon or another chemical neutralizer can help remove the organics that are polluting the water as well.

Why is my aquarium water brown?

This one straddles a fine line. There are some environments known as “blackwater” systems where this is actually a good thing and completely natural. These environments are usually in forested areas without a lot of water flow. The leaves, wood and other organic matter in the water releases a substance known as tannic acid that dyes the water brown…this is the same thing that makes the tea you drink turn brown. Some fish that live in these environments actually need this kind of water chemistry and there are additives and materials available to help aquarists create this kind of system. If you don’t have these fish and don’t want a blackwater tank however, it can be an unsightly nuisance. This usually “accidentally” comes about from driftwood in the aquarium that hasn’t been properly pressure-treated or is too soft and replacing that wood will get rid of the source of the color. Carbon in the filter will help with this as well to remove the color and organics from the water. Keep a careful eye on the pH if you are seeing your water turn this tannic brown to make sure that the acids aren’t lowering your pH too far.

These are the most common questions we get about the color of the water in an aquarium. If you are seeing a different “color” or if the solutions here aren’t resolving the problem in your tank, give us a call or comment below and we’d be happy to help you figure it out!

For additional information – check out this article addressing a specific question from a That Fish Blog reader – Clearing Cloud Water.



  1. avatar

    We just got a new tank- been set up 3 days and is milky cloudy- I pulled the fish out and put them in a bucket until we can see the tank clear. NO ONE ever mentioned this cycle as something that would occur, and we thought the filter was not doing it’s job- changed the cartridge, thinking that that would help clear things, but apparently we’re wrong on that issue. Also pulled the plastic plants and moon rock out and cleaned them(w/H20). We do not have someplace, other than the bucket, to keep them in during this cycle. We do not want to endanger them(goldfish). There are no live plants in the aquarium. Should we put the plastic plants & moon rock back in now, or wait?
    Also, no one told us about which filter systems are best (and easiest to deal with)-we are not fish people. This started because the pet store owner passed out goldfish at Halloween last yr. Everyone else’s fish died- not ours…so we got a bigger tank and a couple companions- this darn 50 cent fish now has cost well over $100 (not impressed), and now we’re thinking we best get a gravel floor filtration system, but have no idea which is the best and easiest (the less time I, personally, have to deal with them, the better-I’m a fur animal person, not a fish lover-except for eating them!)
    Please tell us which is the best filter(floor gravel type). Also you mentioned a reverse-type system…how do we do that? Our tank is 10 gal. and came with a filter. We purchased a round sponge type of air pump. There are plastic plants, a moon rock, and glass or plastic rounded button looking things on the floor. Our original tank was a gal. and had gravel, a plastic plant and a small cylinder type air pump.
    How do we test the water(no one mentioned that aspect, either), and is there a test kit that tests for all the things one has to test for- if so, which one is it?
    And last but not least, what else? It seems like we were not informed of so many things about the keeping of the goldfish- We’re wondering how many other ‘surprises’ are coming down the pike? Really think people who give fish away to children ought to tell them the costs involved, and the things that need to be done for the fish BEFORE they hand these kids a ‘gift’.
    Thanks for your informative writings. You can answer me directly at atachaka at springsips dot com
    Laura & family

  2. avatar

    Hi Laura, It sounds like there is a combination of things going on here.

    First, your tank needs to cycle. You can put the decorations and everything back in the tank during this period; they won’t affect it or vice versa. You can read more about this cycling process in our article on The Nitrogen Cycle.

    Our Goldfish Species Profile and Freshwater Aquarium Basics articles and our blog “How To Care For Carnival Fish” may help you as well. A 10-gallon tank is also small for goldfish with the high amounts of waste they produce. The “Carnival Goldfish” that are usually used as prizes like you mentioned are the same Comet Goldfish sold as pond fish and can grow up to almost a foot in length. Keeping a tank that small clean with those fish can be tricky and will need ample filtration.

    You mention that your tank came with a filter…what kind of filter and how powerful is it? I’m not sure what types of filters you are referring to by “floor gravel type”. When you mention “reverse-type system”, are you referring to the Reverse Osmosis Filtration mentioned in this article? That filtration system isn’t for on the tank; that is for filtering water out of the tap or other water source before use in your tank, similar to the Brita water filters or other filters used for drinking water (more details about that here). We have some information on Choosing An Aquarium Filter and the Best Aquarium Filters on our website. Filter size recommendations from manufacturers are generally very conservative and mased on lightly-stocked tanks. For a small 10-gallon tank with goldfish like yours, I would recommend a power filter or canister filter rated for at least 20-30 gallons.

    To test the water quality, you will need a test kit. The most important basics to test are Ammonia, Nitrite, Nitrate and pH. These four are like the “vital signs” of the aquarium hobby and are the most important to test when you see a problem. Goldfish tanks often have issues with Ammonia and pH with the amount of waste they produce so keeping an eye on these will be important. I would recommend getting test kits that use liquid drops rather than test strips; they are far more accurate and will last (and remain accurate) for much longer. You can get a Master Test Kit like this one or you can buy each individually. If you have any local pet stores or even swimming pool stores, they may test water for you as well…we test water at our retail store any time at no charge.

    Hope that helps…feel free to let us know if you have any more questions!

  3. avatar

    Hi. I bought a 20 gallon aquarium three weeks ago and water is still cloudy. I’ve used water conditioner, bacteria supplement, and stress zyme.

  4. avatar

    Hello Yvette, We would need more information about the aquarium to answer that question. I would recommend giving our Fish Room staff a call at 717-299-5691 so we can discuss the problem with you. Some questions that we would need to answer are: What type of aquarium is this – freshwater or saltwater? Have you tested the water and what are the test result levels of pH, Ammonia, Nitrite and Nitrate at least? What color is the cloudiness – white, brown, green, yellow? What types of fish or other livestock is in the tank and how large are they? Have you been doing any maintenance on the tank like water changes? What type of filter is on the aquarium? How long has the tank been set up with something live in it during those 3 weeks?

  5. avatar

    I bought a new 20 gallon tank with a fluval canister 206 filter I set it up yesterday ,my question is will the water clear on its own

  6. avatar

    Hello Gloria, That would depend on the reason why it is cloudy. As we discussed here, cloudiness can have many different causes. If you have a new tank, cloudiness is normal during the cycling processed as we discussed in the “Why is my water cloudy?” section above as your tank goes through the cycling process. I wouldn’t expect that cloudiness to begin until a week or two after the first livestock is added however. If you only just set the tank up and didn’t rinse your gravel or substrate enough, then any dust or fine debris on it can make the water look cloudy or dirty. That cloudiness should clear up within a couple days as the filter removes it but you should keep an eye on the filter if this is the case since it may become very dirty very fast as it removes this dust.

About Eileen Daub

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Marine Biologist/Aquatic Husbandry Manager I was one of those kids who said "I want to be a marine biologist when I grow up!"....except then I actually became one. After a brief time at the United States Coast Guard Academy, I graduated from Coastal Carolina University in Myrtle Beach, South Carolina in 2004. Since then, I've been a marine biologist at That Fish Place - That Pet Place, along with a Fish Room supervisor, copywriter, livestock inventory controller, livestock mail-order supervisor and other duties here and there. I also spent eight seasons as a professional actress with the Pennsylvania Renaissance Faire and in other local roles. If that isn't bad enough, I'm a proud Crazy Hockey Fan (go Flyers and go Hershey Bears!).