Why Does Aquarium Gravel Turns Brown? (How To Clean It?)

Image of an aquarium with brown algae on gravel

Aquarium gravel turns brown because of brown algae bloom in the tank. Brown algae grow due to excessive levels of silicates, nitrates, and phosphorus in the tank water. They also grow because of poor lighting, inefficient filtration system to clean the aquarium water, and irregular tank maintenance.

Let’s now discuss the various causes of brown algae and how to prevent them.

5 Main Reasons Of Brown Algae Growth In An Aquarium

Brown algae are natural in an aquarium ecosystem. A lot of aquarists face brown algae bloom in their fish tanks.

So, understanding the primary causes of brown algae is essential.

Given below are some of the main reasons for brown algae growth in an aquarium.

1. Silicate in the tank water.

Silicate is the most common reason for brown algae bloom in home aquariums.

This is because silicates are diatom’s favorite food.

And since diatoms make up brown algae, they easily overgrow in a tank rich with silicates.

Now the question is, what are the possible sources of silicates in an aquarium?

There are two primary scenarios in which silicates end up in the tank water.

  1. Water source: Many water sources, including tap water, contain silicates or silicic acid, or compounds that contain these elements. These elements break down and release silicates into the tank water.
  2. Substrate: If you have a substrate like sand or others that are rich in silicate compounds, it will leach out and enter the tank water. Once the silicate builds up to sufficient levels, brown algae will appear in patches in the aquarium. If the brown algae are not cleared immediately, they will cover the entire tank within few weeks, or sometimes even sooner.

2. New tank setup.

Brown algae are a common occurrence in many newly set up tanks.

When the tank is newly set up, all the parameters are not stable.

So during the initial few days, beneficial bacteria start establishing themselves to create a stable environment.

However, the beneficial bacteria take some time to colonize and absorb the extra nutrients available in the tank water.

During this time, the chances of brown algae bloom are high.

However, as the silicate supply is limited in the tank water, they don’t thrive long.

Once the silicate starts depleting, the brown algae start clearing by themselves.

3. Increased levels of nitrates and phosphates.

Increased levels of nitrates and phosphates contribute toward brown algae growth in an aquarium.

Nitrates and phosphates are essential nutrients that fish require to perform their bodily functions.

However, if their levels increase, it contributes to the growth of brown algae in the tank.

Potential sources of nitrates in the tank include:

  • Uneaten fish food,
  • Fish waste,
  • Decaying plant matter,
  • Plant fertilizers, if used for plant growth,
  • Tap water and well water, etc.

On the other hand, phosphates are mainly caused by the wastes in the tank as they break down in the aquarium water.

Some other sources of phosphates are:

  • Dead fish,
  • Fish waste,
  • Uneaten fish food,
  • Plant decay,
  • Dead algae, etc.

4. Poor lighting.

Poor lighting is another reason for brown algae growth in aquariums.

Interestingly, most types of algae need excess light to grow. In contrast, brown algae thrive even in low lights.

However, aquarists worldwide have different opinions on whether low light or excess light is conducive for brown algae’s overgrowth.

This is particularly evident in large tanks where some aquarists purposefully grow algae in their tanks.

In those tanks, you can notice more green algae in the well-illuminated parts of the tank.

On the other hand, brown algae are primarily visible in the part of the tank where lighting is low.

5. Inadequate filtration system.

An inadequate filtration system is yet another contributor to brown algae overgrowth.

Aquarium filters purify the water by trapping the fish waste, uneaten fish food, decayed plant matter, and other debris.

This helps reduce the ammonia levels in the tank water.

If the filters are not efficient, they will spike ammonia and nitrate levels in the tank water, fueling brown algae growth.

How To Clean Brown Algae From Gravel?

The technique to remove brown algae from the gravel depends on the gravel’s size.

If you have large rocks as gravel, you can remove and clean them separately.

In contrast, for smaller rocks, the gravel vacuum is the most efficient way to get rid of brown algae.

You can vacuum the gravel with a siphon. Vacuuming is crucial as it prevents brown algae from growing back on the gravel.

You need to vacuum only the top layer and surface of the gravel.

Otherwise, you risk removing the beneficial bacteria that are needed for biological filtration.

Moreover, the absence of beneficial bacteria further worsens the brown algae problem.

Also, aquarium gravel washers or siphon kits are readily available at pet stores and are also simple to use.

You can follow the below-mentioned steps to vacuum the gravel for removing brown algae:

  • Place a clean bucket near the aquarium. You should place the top of the bucket right below the aquarium.
  • Submerge the entire hose in the aquarium water. You can hold your thumb tightly over the end of the hose so that water doesn’t escape.
  • Then, lift the end of the hose out of the aquarium and lower it into the bucket.
  • Remove the thumb from the end of the hose so that water begins to siphon from the aquarium.
  • You can then hold the cylinder vertical and start vacuuming the gravel by using an up and down motion.
  • Once the gravel starts moving, some gravel may rise into the tube. However, it will fall into place once the tube is lifted.
  • This process separates the brown algae from the gravel. It then becomes easy to clean the brown algae.
  • After completing the vacuuming process and removing the brown algae, you can refill the aquarium with water that matches your tank conditions.

How To Prevent Brown Algae From Coming Back?

Brown algae occur naturally in many newly set up tanks as the tank’s system needs time to mature.

However, it can also overgrow over time if no preventive measures are taken at the right time.

The good thing is, you can easily prevent brown algae bloom by diligently following the below-mentioned guidelines.

  • Regular maintenance of the aquarium.
  • Frequent partial water changes.
  • Appropriate lighting, not less than 8 hours a day.
  • Installation of efficient filters that maintain the water quality in the tank.
  • Water movement to discourage diatoms along with good filters. The water movement helps filter the tank water. Additionally, it also assists in preventing the diatoms from attaching to the gravel and other aquarium surfaces.
  • Usage of reverse osmosis (RO) water after water changes. This water is free from nitrates and silicates, which are the primary reasons for brown algae overgrowth.
  • Optimal levels of nitrates and phosphates in the tank water.
  • Avoiding overfeeding the fish. If you overfeed the fish, it will lead to excessive waste production. Excess waste produces high levels of nitrites and nitrates, which encourage brown algae growth.
  • Avoiding all sources of silicates like a salt mix, live sand, and silica sand that can help brown algae bloom in the tank.

Are Brown Algae Dangerous To Fish And Plants?

Brown algae are not dangerous to fish as long as they are under control. Some fish even prefer to eat brown algae.

At the same time, you can see other fish happily swimming around the brown algae.

Moreover, if you notice that your tank inhabitants are more active in the presence of brown algae, it’s a clear indication that the oxygen level in the tank water is low.

Further, brown algae help the fish to thrive. The diatoms that make up brown algae consume carbon dioxide and release oxygen.

So in a way, they help oxygenate the tank water.

Now, although brown algae are suitable for fish, they are detrimental for aquatic plants.

This is because brown algae thrive on the same nutrients that plants need.

So they compete with the plants for the essential nutrients.

Moreover, brown algae also block the sunlight that plants need for optimal growth.


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