Guide: Cloudy Aquarium Water

Why is my tank cloudy?

Clouding is caused by microscopic particles floating around in your water, which make your water opaque.  There's no one substance called "clouding" - all clouding is microscopic particles.

What caused this clouding?

Lots of things can cause particles to form in the water, because there are lots of types of particles.  Identifying these particles is the first step to figuring out what caused them and making them go away.  Here are the four main types of clouding:

image.pngFree-floating bacteria 

Bacterial clouding tends to be white, gray, or yellow, and usually appears in new tanks or after a major change.

Free-floating bacteria in the water feed on dissolved organic waste.  Usually their population is low enough that they cannot be seen, but when there is a buildup or sudden spike in available organic waste, bacteria growth spikes, leading to visible clouding in the water. 

 Factors which stress beneficial bacteria colonies like unstable water parameters or insufficient bio-media will exacerbate this issue further, and maintaining healthy biological filtration will tend to prevent bacterial clouding.

mceclip1.pngFree-floating algae

Algae clouding is green and usually appears in tanks with high light, phosphate, nitrate, or all three!

Free-floating algae is almost always present in the tank but invisible to the naked eye.  However, when there are high levels of nutrients like phosphate and nitrate available along with extremely strong light, algae populations can bloom to the point that they turn the water green.


Precipitation can be any color, but usually appears immediately after a supplement or buffer is added. It usually only appears in tanks with a very high concentration of buffers or minerals such as reef tanks, but can also appear in high-mineral freshwater tanks or as a result of intentional use of a precipitant. 

If there is a severe imbalance in your water chemistry, minerals and other materials can start precipitating out and causing notable clouding.  This is particularly common in reef tanks.  Test your water to ensure major water parameters are within their proper range!


Dust clouding can be any color, and appears as a result of adding filer media, decorations, or substrate capable of producing dust particles.

Sometimes the solution is simple!  Dust from substrate or filter media can cloud water, and will usually settle out of the water given a few hours.

How do I get rid of clouding?

No one likes looking at cloudy tanks for longer than absolutely necessary, so clear it out!

mceclip6.pngFine Filter Media

Regardless of clouding type, you’ll need fine filter media like filter floss to pull the particles out of the water.  Pack the filter floss into your filter so the water is forced to flow through it.

Water Clarifier

Dose Clarity® to clump the particles that are causing clouding together so that the fine filter media can remove them from the tank.


Once your filter floss is saturated and your water is clear, remove it from the filter and either thoroughly rinse or replace it.  Particles from clouding can clog filter media to the point that it obstructs flow through the filter.

How do I keep the clouding from coming back?

Now that you’ve cleared the water, you’ll need to prevent the clouding from coming back. Bacteria and algae clouding tend to grow back if conditions are right, and both precipitant and dust clouding will appear again if steps are not taken to prevent them.

Stabilize water chemistry

New aquariums are very prone to clouding due to unstable water chemistry.  Ensure that your pH, KH, and GH are stable and in the proper range by using buffers like Alkaline Buffer™ and Acid Buffer™.

Reduce nutrients

Using filter media like Purigen® or MatrixCarbon™ can help lower levels or organic waste which feed free-floating bacteria.  Likewise, the use of media like PhosGuard™, PhosBond™, or PhosNet™ reduces phosphate which can feed free-floating algae.  Keep in mind that planted tanks require phosphate to support plant growth and some kinds of freshwater tanks are buffered using phosphate. Seachem® does not recommend using phosphate absorbers in these types of systems.

Support beneficial bacteria

Underdeveloped or unstable beneficial bacteria colonies are one of the primary causes of clouding.  Avoid things that stress or overwhelm beneficial bacteria colonies like unnecessary medication or over-feeding, and make sure they have plenty of bio-media like Matrix™. 


Some of the most common issues that lead to tank clouding are overenthusiastic filter cleaning, gravel siphoning, and massive water changes.  Regular tank and filter maintenance is essential for treating and preventing clouding, but too much will lead to instability of the beneficial bacteria colonies and water chemistry, making the issue worse.  Patience and consistency are your friends when dealing with clouding!

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