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Aquarium Gravel Is Turning BLACK: What Exactly the Solution Is

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Lots of aquarists may notice that their aquarium gravel may turn black after a while. This destroys the attractiveness of the aquarium. If you are new to this hobby then you should aware of this

The most possible reason why aquarium gravel is turning black is the lack of oxygen in the aquarium. Gravel may turn into grey or black colour in stagnant water. Besides this, the Black beard algae are another possible reason for black coloured gravel. Gravel may change its colour due to fish waste also. 

Now only these two line answers will not give you clarity about it. Let’s jump into the topic to know more.

Why is my aquarium gravel turning black? 

An aquarium may get several attacks if it is not maintained properly. The blackish gravel is one type of symbol that the aquarium is not healthy anymore.

Less amount of dissolved oxygen

This is very tough to interpret the causes of the gravel’s colour change. Your aquarium gravel may be sedimentary rock. This type of rock changes its colour along with the oxygen level in the water. Generally, the greenish colour of gravel means there is continuous water flow and a sufficient amount of dissolved oxygen is present in the water. If aquarium gravel is turning grey or black colour that means the water is stagnant and very little dissolved oxygen is present in the water. Water circulation may be almost zero at that point. The reddish colour of aquarium gravel means there is an excessive amount of dissolved oxygen in the water.

For another reason, the colour of gravel may change. This depends on the iron percentage. If the ferric iron is present in aquarium gravel then it may change its colour into purple or red or yellow gradually. The colour of gravel may turn green if ferrous iron is present in gravel.


There is not the so-called proper and right solution. You should change the water daily. If your aquarium has a lack of water circulation then you should change the filtration system. You can use air stone for it. Airstone helps to circulate the water vertically. This is very good for the substrate of the aquarium.

You should remove unnecessary decorative materials from the water. These materials generally create dead spots here and there in the aquarium. You must assure that every side of your aquarium is getting oxygen from proper water circulation.

Black beard algae       

Your aquarium may face algal bloom due to excessive nutrition level. Black beard algae or BBA is one of the worst algae that your aquarium may get. If your aquarium gravel is turning black means there is a high possibility that your aquarium is affected by black beard algae.

Causes of appearing black beard algae

Generally, algae grow more due to excessive nutrient level in the water column. Black beard algae grows also just because of this reason. If your aquarium has an imbalance of iron then also black beard algae may appear. The iron level in the water should be between 0.05mg/l to 0.1mg/l. Excessive amounts of iron may lead to black beard algae too. If you are using iron fertilizer then you should reduce the dosage immediately.

Black beard algae are responsible for the low pH level of the aquarium. So if your gravel is turning black with a low pH level then you should know that your aquarium has introduced black beard algae.

Water has OHion. The free carbon dioxide forms bicarbonate (HCO3-) with it by chemical reaction. Now black beard algae take carbon molecules and release hydrogen into the water. Due to excess hydrogen, the pH level starts to decrease. (CO2 + OH → HCO3)

Due to this chemical reaction (biogenic decalcification) as precipitate calcium carbonate forms. This calcium carbonate is accumulated on the black beard algae gradually and this becomes non-attractive to algae eaters. So ultimately algae eaters also don’t like to eat these algae.

Medication solution of Black Beard Algae 

To control you can use a medicine named EasyCarbo. This is introduced by Aquasabi, a trusted source to know aquarium related information like us. We are recommending you after using this medicine. The dosage of EasyCarbo is 1-2 ml per 50 litres of water.

Another cheap solution is adding hydrogen peroxide to the aquarium. This is such a chemical that destroys BBA within a few days. First, the BBA turns soft and it turns green gradually. The dosage of hydrogen peroxide should be 1ml/500ml of water.

If black beard algae have appeared in your fish tank then this is almost impossible to revive the fish tank again. Your tank will definitely be destroyed. Fish will not be affected by black beard algae. But your aquatic plants will be all dead due to their medication. You can’t remove BBA fully without these medication processes.

Read more:- How to remove algae from aquariums? Solutions for each type of alga

Excessive fish waste

Fish waste accumulation is not good for any aquarium. This may increase the ammonia level suddenly. Due to this high ammonia level fishes may die off. Not only that, but due to this excessive fish waste, the aquarium gravel may change its colour due to ammonia or urea. This provides a symbol of a bad tank condition. So you must take action against it.


To remove fish waste you can rely on a good filter. You should take such a filter that can do biological, mechanical and chemical filtration altogether. The canister filter is the most suitable filter for all types of fish tank. But the cost of this filter is comparatively high. So you can use the HOB filter to get three types of filtration in the aquarium.

Sponge filters are also good for this type of situation. This mainly does mechanical filtration. After a while, this filter starts biological filtration of the fish tank.

Weekly water change can help a lot to get rid of this problem. You can do a 20% of water change twice a week. If you are already doing that then you must increase the rate of water change.

Read Here:- Sterilization of aquarium after fish’s death: A VETERINARY guide 


Turning in black colour of aquarium gravel is not a suitable condition for the fish tank. This may seem like nothing is happening except colour change but this is the starting of a bad condition of a fish tank. This is very tough to know the exact cause of this issue. But this article has covered some possible reasons for the colour change of aquarium gravel.

Black beard algae are very bad for a fish tank. This may destroy your well-decorated planted fish tank very quickly. So you must be aware of it. The deficiency of oxygen means the fishes are not well enough. So you should arrange the oxygenation. From this discussion, this can be concluded that you should be aware of your aquarium if the gravels are turning in black colour.

So best of luck & happy fishkeeping 🐟

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