Why Does Aquarium Sand Turns Black? (How To Clean + Prevent It?)

Image of an aquarium with sand turning black

Most fish tank owners are familiar with the sand turning green or brown. However, it may even turn black at times. This issue may become persistent and keep coming back. So, why does aquarium sand turns black?

Aquarium sand turns black because of the presence of black algae. It can also turn black when the leftover food and organic waste produce anaerobic bacteria and fungal colonies or if the dye washes out of the sand. Moreover, high iron and manganese levels of water can also turn aquarium sand black.

Let’s dig deeper into this now.

4 Main Reasons Why Aquarium Sand Turns Black

It can be very frustrating to find black sand in your aquarium. In most cases, the black color appears as a patch, and it quickly spreads and takes over the entire substrate.

Fortunately, the color is very noticeable, and you can’t miss it. By intervening quickly, you can prevent it from getting out of control.

Given below are the main reasons why aquarium sand turns black.

1. Presence of black beard algae.

Black beard algae are a variety of red brush algae. They occur in pools, ponds, and artificial water habitats.

These algae usually reach aquariums by hitchhiking on aquatic plants through contaminated tank décor or substrate.

They look like mold but quickly grow hair-like structures when left to grow.

Black beard algae can make a tank look very ugly and is very persistent. Even if you remove it, it may return.

2. Anaerobic bacteria and fungal pockets.

Leftover food and organic waste in a tank can reach the substrate and settle into the layers of soil. If you don’t get rid of these particles, they will encourage anaerobic bacteria and fungal growth.

These microorganisms expel toxic gases like hydrogen sulfide. They also form black patches in the sand and make it look ugly.

You can identify the presence of anaerobic bacteria and fungal colonies by their odor. When you disturb this sand, it will stink and make the water cloudy.

3. Dye discharge from treated and colored substrate or decorations.

If the sand, gravel, or rocks in your tank contain artificial colors, the dye may wash out of it after a while.

As the dye gets into the water, it can stain parts of the substrate. So what you see could be artificial colors staining the sand in your tank.

4. High levels of manganese and iron in the tank water.

Increased levels of iron and manganese in tap water can cause black sediments to appear. These can attach to the sand and make it black.

Since neither of these substances is harmful to aquatic plants or animals, you can ignore them.

Nevertheless, it’s wise to check with your local water department or use testing kits to confirm this is indeed the cause.

Recommended Further Reading:

How To Prevent Aquarium Sand From Turning Black?

The good news is that you can prevent aquarium sand from turning black by promptly identifying the cause and addressing it before it gets out of control.

Proper tank maintenance and cleaning will help you prevent most issues. It will be difficult for algae and other microorganisms to establish themselves and grow in a well-maintained tank.

Algae growth and the formation of anaerobic bacteria and fungal pockets in the substrate are more likely in dirty and poorly managed tanks.

Regularly vacuum the substrate and use an efficient tank filter to capture and eliminate waste particles and dirt.

Frequent water changes will also help you keep your tank hygienic. Moreover, clean your tank decorations to prevent them from harboring germs and parasites.

You should also pay attention to the water chemistry to check for likely problems. High levels of phosphates, nitrates, manganese, and iron can make the sand turn black.

Regularly monitor the chemical levels of your tank to address any imbalances. With due diligence, you can prevent it from spreading and spoiling the entire substrate.

How To Clean Black Color From Aquarium Sand?

Black patches or spots in the substrate usually require timely intervention.

Unlike when your aquarium sand turns brown, it will not resolve on its own. So unless you address it promptly, your entire sandy substrate can turn black.

This will compromise the health of your aquatic pets and plants and make the tank an inhospitable environment. Hence, it’s crucial to identify the cause and address it without any delay.

Let us look at each of the potential causes and what you can do to prevent it from getting out of control.

How To Get Rid Of Black Beard Algae?

Black beard algae thrive in moist, unhygienic tanks with excess nutrients in the water. They feed on nutrients in the water.

They can easily overwhelm an entire tank and destroy its visual appeal if left unchecked.

Fortunately, you can eliminate black algae from a tank with the following steps.

1. Adjust the phosphate level of your tank.

Black algae grow well when the phosphate level of the water increases. Phosphates can increase if there is leftover food or fish waste in the tank water.

The tap water in some places also has high phosphate levels. To confirm it, test the water using a suitable kit. If it’s high in phosphates, you can fix the problem in the following ways.

  • Switch to distilled water or RO water instead of regular tap water.
  • Use phosphate-absorbing media in your filter to remove phosphates from the water.
  • Remove any uneaten food and leftover organic waste from the tank to reduce the phosphate content of the water.

2. Treat the affected surfaces with hydrogen peroxide.

Over-the-counter peroxide of 3% strength can eliminate black algae. Wash the affected sand with this solution to eliminate all the black algae.

You can also soak the affected decor items in the same solution and use it to wipe surfaces with black mold.

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3. Inject CO2 into the tank to enhance plant growth.

Insufficient CO2 levels will compromise the growth of aquatic plants and create a hospitable environment for black algae.

Inject CO2 into the water to improve the health of aquatic plants. When these plants thrive, they will outgrow the black algae and starve them of the nutrients they need for survival.

4. Add algae-eating fish and animals to the tank.

Some fish and shrimp eat algae. Introduce these animals into your tank to eat all the algae and keep it from growing and spreading.

Siamese algae eaters are some of the best fish to vanquish black algae. Plecos, black molly, goldfish, catfish, and some other types of fish also enjoy eating black algae.

Amano shrimp will also feed on this type of algae and keep it from growing. So, add these animals to your tank, and they will take care of the entire problem.

5. Perform a temporary black-out.

Like all plants, algae need sunlight for photosynthesis. The lack of light will starve them and reduce their chances of survival.

So, if nothing seems to solve your black algae problem, subject your tank to a complete black-out for three full days.

This will also affect the rest of your aquatic plant life. However, the effects will be temporary. Your other plants will bounce back once you restore access to light.

However, since the algae are small, they will be unable to withstand the lack of light and perish.

How To Prevent Anaerobic Bacteria Colonies And Fungal Pockets From Turning Aquarium Sand Black?

Black spots in the sand can sometimes indicate the presence of anaerobic activity due to bacteria. These bacteria produce hydrogen sulfide.

Hydrogen sulfide causes offensive odors and are toxic in large quantities to your aquatic life.

If you disturb such anaerobic pockets in the substrate, you will notice gas bubbles rising into the water and popping. It would turn the water cloudy and make it smell like rotten eggs.

To get rid of anaerobic bacteria, fungus, and other pesky parasites, you should always be mindful of tank cleanliness.

Remove any uneaten food and organic waste from the tank. Leftover food and organic waste can rot in the tank.

Due to the compact nature of sand, they will usually not settle into the lower layers. So, most problems will be limited to the upper layer of the sand only.

Nonetheless, any particles that reach deep into the sand will disintegrate and produce harmful ammonia and nitrates. These chemicals will make the water toxic and harm aquatic plants.

Also, clean the substrate regularly using aquarium vacuum gravel. Stir up the sand to prevent the formation of anaerobic pockets. This will prevent the gas from forming and also dislodge the colonies.

Interesting Further Reading:

How To Prevent Dye Discharge In The Substrate From Turning Aquarium Sand Black?

Sometimes, dye discharge from gravel, colored rocks, or the sand itself is causing the discoloration of your tank substrate.

In this case, you may be able to fix the problem by just vacuuming the top layer of the sand. The dye that settles in the upper layers will come out of the tank.

If you are not satisfied with the result, you may like to consider replacing the upper layers with fresh sand.

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