Why Does Aquarium Gravel Turn Black? (2022)

Why Does Aquarium Gravel Turn Black? Is it Dangerous?

Aquarium substrates are core elements of your tank set up. They provide more than just making the bottom of your tank pleasing to the eye.

Your substrate is vital for the health of your tank plants and your fish. Choosing the right substrate can be confusing. However, once you see your substrate begin to go a darker color, it can be worrying. This leaves many tank owners asking.

Why does tank Aquarium gravel turn black? Is it dangerous?

One of the main reasons tank substrate goes black, is from what they call ‘Black Beard Algae.’ What begins as a black coating will grow to hair-like strands. A couple of reasons can cause this, such as infected items introduced into the tank, or the water has carbon dioxide levels that are too low, and too much light. It isn’t dangerous as long as you tend to dispose of it sooner rather than later.

Here you can learn what causes it, and how you can quickly get rid of it using various methods.

Aquarium Gravel turning black

What is Black Beard Algae and What Causes It?

Black beard algae (brush algae) has a soft and slippery feeling that resembles fur. While called Black Beard, it is part of the red algae family. You see this grow on hard surfaces and plants in dense patches.

If you look closer, what appears black is a blackish-green in color in comparison to red members of the family. It is almost impossible to remove by hand, and not to eliminate it correctly means it will come back quickly.

For the most part, this algae isn’t harmful, and you can find it eaten by some tank dwellers. The primary culprits for the introduction of this are plants that are already infected. Besides, your tank may face exposure to too much light, and there is a reduced level of carbon dioxide.

  • CO2: One reason for low levels of carbon dioxide can come from an underpowered tank filter. If there is poor circulation, then CO2 levels will be low, and the ideal environment quickly appears in an aquarium.

If this is the current state of your aquarium, then adding more CO2 ought to be the number one priority. You can achieve this through a couple of ways, which you can learn later.

  • Light: Everything inside an aquarium needs light to some degree to survive. However, as soon as it is too much, then you can suffer an algae outbreak. One thing to note is, the more the amount of light in your aquarium, and the more extended the periods, the faster the spread of algae you will encounter.

Often this problem occurs when tank owners are delivering light to their plants. It happens they are unaware they are also providing the perfect conditions for algae to bloom. As such, forgetting to turn off aquarium lights leads to the same issues.

If your algae issues are because of light, you can restrict its use, or leave your lights off for a couple of days, to get the algae growth back under control. Using aquarium lighting timers are the ideal way to keep lighting at the right levels.

how to get rid off Black Beard Algae

Will Black Beard Algae Harm Fish and Plants?

Black Beard algae will not generally harm fish by itself. However, the more it takes over aquariums and covers your substrate, plants, and any other surfaces, and it can begin to affect water conditions.

Once it reaches a certain level, you may find the nutrient levels affected, and this can begin harming your tank.

If you have live plants, then it may be the end for them if you don’t attend to them quickly. Like your fish, the algae won’t kill them directly, yet it smothers them. Over time, your plants lose the ability to photosynthesize, and there will be competition for nutrients in the water.

Controlling and Removing Black Beard Algae

While this algae growth can be nearly impossible to remove manually, you can use several methods to bring it under control and to rid it from your tank.

Hydrogen Peroxide

You can find two ways to treat your tank, and this will depend on it the algae is just beginning and covering your plants or other surfaces, or it has reached your substrate and is taking over your tank.

To treat separate items and plants, you need to soak the affected objects in 3% hydrogen peroxide for around three minutes before rinsing in fresh water.

It is possible to purchase 3% hydrogen peroxide over the counter, and you can use it for many more conditions around your aquarium. Treating individual items can be hard, and it won’t help to treat your substrate if that is showing signs of algae growth.

Luckily, it is as easy to treat a tank with hydrogen peroxide, as it is to tend to individual plants and items.

Following this method, you ought to be able to clear all signs of Black Beard algae from your tank within a few weeks. Before commencing this, you need to eliminate a slow water flow as one of the causes. If your filter is underpowered or underperforming, you will need to think about either purchasing a more robust aquarium filter or adding another filter to boost the amount of water flow.

  1. First, calculate the number of gallons your tank holds
  2. Add 10mls for every 15 gallons of water. Be sure to add to flowing water
  3. Repeat the adding process for three days

At this stage, you can wait to see what happens, and if you begin to see the algae fade in color, you are on the right track.

If everything has gone according to plan, your algae issue will be clear inside a month. If you find your tank is still suffering, you will need to repeat the process with a slightly higher dose than the initial 10mls.

You can notice plants may fade as well as the color of the algae, although they will quickly bounce back from this.

All your fish and other inhabitants should be fine during these steps, yet if you are unsure, you can transfer them to a separate tank while you deal with your algae problem.

Boosting Carbon Dioxide Levels

Increasing these levels won’t kill these algae, although it can help prevent its presence. An increase in CO2 will aid plant growth, and as these grow, there are fewer nutrients available for the algae. It is possible to increase these levels in one of two ways.

CO2 injection is quick and direct and can range from a CO2 aerosol to large cylinders. Second is in the form of liquid carbon and can be found in such products like Seachem Flourish Excel.

Rather than increasing CO2, these contain a kind of herbicide named Glutaraldehyde, which goes to work on your algae directly.

While it is effective, you need to be careful with this, as it can kill fish and everything else in your tank if you add too much

Phosphate Buildup in Aquariums

Phosphates won’t lead to the buildup of these algae; however, they will benefit its growth. Any waste in your tank that settles on your substrate can decay and quickly turn into phosphates. To help avoid this situation is one step closer to prevention.

  1. Don’t overfeed fish
  2. Remove dead fish or other dead tank inhabitants
  3. Remove dead plant matter as quickly as possible
  4. Carry out regular water changes

To remove the matter from your substrate, you may need a good gravel vacuum. However, once you have a filter that is suitable for your tank, you will see phosphate levels reduce.

Phosphate Buildup


Any algae growth can be unsightly, yet because of your substrate and any other surfaces become black with this one; it can take on a murkier appearance.

You will find out you only have this issue if you don’t tend to your aquarium in the right manner, and it is the direct result of poor water conditions.

Once you remove it using the steps detailed earlier, you then have a blank canvas and should ensure your aquarium remains as clean as possible for the health of your fish.

It will also save you hours of effort in ridding the algae when it comes back.