Black Mold in Fish Tank: 9 Symptoms and Treatment

I will talk about what symptoms black mold can present when it infects a fish tank and how to treat the problem for good!

What is black mold or algae?

Black alga is a type of algae that occurs in many forms but generally has darker colors to give it the “black” name.

The black species can be a problem for aquarium owners because they do not always have the same characteristics as other types of algae, and they are difficult to control.

Problems With Black Mold in Fish Tank

Some problems with black algae could include erosion of cement structures, contamination from organic materials from the bottom of the tank to rocks or glass surfaces and lethal toxicity for shrimp and invertebrate pets.

Suppose there were a lot of these harmful toxins. In that case, it will increase filtration time before entering the water back into the middle section of the tank, creating more biomass for bacteria culture.

So more work on the filter means less work on fish tank maintenance. An old and dirty filter often causes black algae in fish tanks. 

How black algae forms? 

Upon death, organic matter such as plant growth or organisms releases so much methane gas that it rises to the water’s surface.

Some of these particles are organic molecules called lipids, including oils, phospholipids, glycerides, and fats.

The compounds’ chemical bonds are broken up when they are exposed to reactive gases in sunlight or when they get trapped between sand grains on the seabed.

The idea behind this reaction is that light energy helps break down bonds among water molecules.

While sand finds particles with lipids attractive because their shapes help form what’s known as a Langmuir film.

How do I know if my fish tank has black mold?

The largest indication of black mold in a tank is usually the smell. If you have gone through the other stages and have not found any.

You can try mixing some bleach into your protein skimmer for 24 hours to see if the smell dissipates.

Symptoms of black mold in fish tank can vary, with the most common symptoms being the following:

  • Black or dark green spots on leaves, plant trunks, gravel, decorations, etc. 
  • clumps on rocks, gravel, and other surfaces
  • A slimy substance covering surfaces 
  • Fish acting strangely or trying to eat slime off objects 
  • Clumps of slime fall from plants into the water column and 
  • Slime clogging filters and other equipment
  • cloudy water (often caused by increased organic content)
  • slime coating on all aquarium surfaces (fish tanks typically have good bacteria that will keep them clean if no chemicals or acids are added to the tank)
  • Discoloration of plants and animals within the tank.

It is typical for owners of an algae-infested fish tank to feel frustrated and distressed.

It’s important to remember that what we’re looking at isn’t dangerous, just unenjoyable to look at or deal with. 

Why is it important to remove the black mold from my fish tank?

Black mold serves as a nutrient for other types of fungus, which will quickly overtake a fish tank and there is no way to get rid of the black mold without getting rid of the other fungi it supports.

Moreover, certain types of fungi can produce toxins that are lethal to the fish’s nervous system.

For this reason, many experts recommend removing any large patches of mold before deciding upon what to do with your fish tank.

What Conditions Aid the Growth of Black Algae?

Environmental factors that promote the growth of algae include a high water temperature, a lack of sunlight, and a lot of nutrients. Keeping these conditions from occurring will help control algae growth.

Some species produce toxins that are harmful to humans and animals that come into contact with algal blooms.

If they aren’t changed soon by climate change, for example, some new species may become dominant due specifically to their ability to thrive in these conditions.

Increased competition among species or increased predation can also lead to decreased populations among weaker ones.

Algae feed on bacteria living on rocky surfaces, which releases benzoquinone as a by-product.

When it’s decomposed, it can release hydrogen ions into the water, which decreases the hardness of the water.

It is bad for fish health because they need minerals to build their protective scales and skeletons! 

Five common conditions are noted below:

High levels of nutrients such as salts and nitrates winds with speeds of less than 15 miles per hour 

Algae-rich water:

An ideal supply of murky and inefficiently disturbed waters will likely produce black algae to a much greater extent than normal.

Warm temperatures:

The warmer the water, the more bustlingly they will go about their business, taking up almost all of the available resources they might need.

These higher temperatures also create conditions that allow growth for other types of algae, such as duckweed.

Active turbulence in turbulent environments:

Turbulence is required for mixing, invigorating, and densifying the water column;this is most often achieved through waterfalls, strong water pumps, and air stones.

Green algae in fish tank good or bad

Many aquarists used to think that green algae in a fish tank were natural and not harmful, but it’s now known to be both.

Some green algae have been found beneficial for the environment of a healthy aquarium ecosystem.

Still, most often, green algae growth will contribute to nitrate pollution and reduce dissolved oxygen levels in the water column.

It means nitrates from decaying green algae cells can accumulate indefinitely and continue producing toxins even after an aquarium degradation has been managed.

There is now evidence suggesting that these common forms of algae compete with invertebrate species.

Such as shrimp or snails for food sources at night when plant life begins to respire more rapidly.

How to Get Rid of the Black Mold in Your Fish Tank?

It is advised that you begin by collecting all of the mites in your fish tank using a net or other mesh to get rid of black mold in your aquarium.

If you have any plants that were also brought into the tank, you must remove them and trim off any slimy plant surfaces infected with mycelium.

Then, thoroughly clean and scrub everything both inside and out to eliminate any fungi spores left behind.

Before or after cleaning your tanks, ensure you sanitize the equipment and anything else in contact with water.

A general rule of thumb when contemplating adding new plants to a tank full of existing ones is to ensure they are not so close together as to promote matting, which can result in a myriad of problems in and of itself. 

Removing mold and algae from a fish tank can be done in different ways.

If you have black mold growing in your aquarium, it’s best to use hydrogen peroxide (bleach) with a non-scented or clear label product.

Avoid using scented bleach, such as lemon scent, because this will produce toxic fumes the animal cannot breathe in.

Keep them away from direct contact with bleach and take cover, since bleach releases harmful vapors when it mixes with water.

Add one cup of alcoholic strength hydrogen peroxide (30%) to one gallon of tank water and mix well.

Remove all animals, plants, and devices out of the container before mixing the solution, since they could absorb the lethal amount of vapors through their gills and tissues.

So the first thing you should do is take your fish out of the tank.

The second would be to clean any other decorations or cover them with plastic to protect them from chlorine.

Finally, use a syringe to put 1 cup of chlorine bleach into 3 gallons of water for freshwater.

Next, completely drain all the tanks, including suck-up hoses, but not the filter or heater.

It will ensure that no water mixes in with bleach so that it won’t ruin those elements.

Then put everything back together, place your fish back in their tank, and rinse off any surfaces you think may have been affected by contamination.

After one month or if there are still sporulating bacteria visible on glue boards, then repeat treatment.

Preventing Black Mold in Your Fish Tank

There are two primary steps to take to prevent black mold in your fish tank. 

First, decrease the number of fish you keep to maintain good oxygen circulation.

Overstocked tanks promote stagnant water with little oxygen, whereas less-stocked tanks provide more than enough room for gas exchange. 

Second, use activated carbon filters on your tank’s intake line and spray down any algae buildup on the outside of the glass panel every day.

It’s also a good idea while maintaining these two points. Always to ensure that there is some type of scrubber system present in your aquarium.

That enables better surface agitation while still keeping all other nutrients away from bacteria growth on surfaces not typically desirous of freshwater life (such as food consumed by the fish).

Signs of a Healthy Aquarium

  • The fish appear active and swim around the tank often. 
  • There is no bad smell coming from the tank, such as an odor of ammonia or rotting food
  • Gas bubbles moving will be seen when at least one fish is above or below the water level in the aquarium for at least 3 seconds (bubbles that rise to the surface and dissolve without causing any appearance of normal fish activity).
  • Most people with healthy freshwater tanks will see bubbles occasionally;
  • for saltwater tanks, gas bubbles should occur routinely in all levels of the aquarium (major bubbling visible 24 hours a day is not unusual in most saltwater tanks). 
  • Clear, clean water without cloudy dirt, smoky deposits,
  • or powdery residue on the glass walls is typical of a healthy freshwater tank.
  • Saltwater tanks should have lots of white salt crystals or specs on the glass.
  • Saltwater fish are accustomed to saline water and will not be visibly affected by large amounts of salt crystal buildup.

Fish that will eat black algae

Algae eating fish depend on the type of algae they eat. Examples include, but are not limited to, these species: 

Black Molly Black Mold in Fish Tank
Black Molly Fish
Catfish Black Mold in Fish Tank
Comet Goldfish Black Mold in Fish Tank
Comet Goldfish
Pygmy suckermouth Black Mold in Fish Tank
Pygmy suckermouth
Rosy Barb Black Mold in Fish Tank
Rosy Barb
Veil Tailed Butterflyfish Black Mold in Fish Tank
Veil Tailed Butterflyfish
Siamese Algae Eater Black Mold in Fish Tank
Siamese Algae Eater
  • Comet Goldfish 
  • Veil Tailed Butterflyfish 
  • Black Molly
  • Rosy Barb
  • Pygmy suckermouth
  • Catfish 
  • Siamese Algae Eater

Can Mold in Fish Tank Kill Fish?

Mold is a type of fungus that can grow in fish tanks, especially if the water quality is poor or the tank is not cleaned regularly.

Mold can be harmful to fish, as it can cause diseases, allergies, or even death.

Some types of mold can also produce toxins that affect the fish’s nervous system.

To prevent mold growth, you should maintain good filtration and ventilation, change 25% of the water every two weeks, and remove any organic debris or dirt from the tank.

If you see any signs of mold in your fish tank, you should clean it thoroughly and disinfect it with vinegar or bleach.

Mold in fish tank can be a serious problem, so you should take action as soon as possible to protect your fish.


When algae becomes too dense, it can have adverse effects on your fish’s health.

We recommend using an aquarium vacuum cleaner with a filter cartridge designed specifically for aquariums so that you are not adding any other pollutants into the tank.

Feed them less food if they seem slim or their color has changed drastically.