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HomeBlogWhy is my aquarium water cloudy?

Why is my aquarium water cloudy?

Ever sit down in front of your fish tank to take a gander at your gorgeous fish, only to be greeted by an underwater fog bank? Cloudy water isn't a sign that the aquarium needs to be torn down, scrubbed out, and restarted. Action needed will depend on the cause of the cloudiness. Fortunately, there are only two causes that are common: algae and bacteria.


Generally, the type of algae that causes cloudy water is going to have a greenish tint to it. There are a few primary events that lead up to an algal bloom. Luckily for you, all of them are pretty easily remedied. If you encounter cloudy, greenish water, here's what you should do:
Check to make sure you're not overfeeding.
Uneaten fish food will break down and pollute your tank. Those nutrients have to go somewhere, and they can end up fueling an algae epidemic.
You're not trying to keep too many fish, are you?
Overstocking can lead to many problems, one of which is a build up of fish poo. Fish waste contains a lot of nutrients that algae loves to break down use to grow. Keep your tank stocked at a healthy level to avoid this problem. Cut back on the number of fish you're keeping.
Test the water you're using for water changes for phosphorus or nitrate.
These are the principle components of fertilizer and algae will grow like wildfire when high levels of these nutrients are present. If they are present, consider using filtered water, or treating the water with something that will remove nitrate, such as Purigen.
Make sure your tank does not get too much light.
Lights should only be run over the tank for 10-12 hours a day, and unless the tank is in a dark room or contains live aquatic plants, There's no need to run the lighting when you're not around to look at the tank. Sunlight can be a big problem, as well. Covering the tank with a blanket for a week will help kill off the algae, but this is only a temporary fix. For a more long term solution, you'll need to block incoming sunlight from the window or move the aquarium.
Are you keeping up on your water changes?
If your nitrates are consistently reading over 40 ppm, you probably could stand to do more. Begin to do small, frequent water changes, such as 10-20% 1-2 times a week. Do not do large water changes out of the blue, as this can have some seriously negative impacts on the health of your fish.
Check and make sure all filtration equipment is properly functioning and rated for the water volume of your system.
Change or rinse filter pads and filter media, wash out that canister filter, etc. Over time, gunk can build up and greatly reduce filtration efficiency.


In an aquarium, nitrifying bacteria are generally your friend. These guys break down harmful ammonia, generated by fish and fish food, into nitrite and finally the much less harmful nitrate, which is then removed during a water change or fed on by live plants. If There's a sudden influx of nutrients, or in a new tank when the natural microbiome (population of bacteria) is not established, sometimes bacteria can explode in numbers to the point where they leave the surfaces they normally graze on in your filtration and gravel, and move up into the water column. This sudden influx of bacteria, often called a bacterial bloom, can make it look like milk just got dumped into your aquarium. Generally, bacterial blooms are not harmful and take care of themselves.
  • Make sure all equipment is working properly. A sudden dropoff in water flow can cause bacterial blooms.
  • How old is your tank? New tanks often undergo bacterial blooms as part of the cycling process. This bacterial bloom is harmless and will take care of itself. Trying to clear the water by doing water changes may actually prolong the bacterial bloom and make things worse.
  • Did anything change recently? Adding a lot of fish, removing bacteria by changing substrates, using certain medications that can kill them off, and the like, can cause a bacterial bloom. Give it time and it will go away on its own.
  • If you don't want to wait, products that cause particulates to bind and fall to the bottom of the tank can be used, such as Clarity.
Click the following link to read Aqueon's article with information about cloudy aquarium water in a new fish tank:

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