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Mold In Fish Tank? 10-Step Plan To Remove It.

Mold In Fish Tank? 10-Step Plan To Remove It.

Image Credit: Muffinn (CC License)

Mold in the fish tank? You’re not alone. We’ve all dealt with unsightly mold at some point in the fishkeeping hobby. Sometimes, even when you dot all the i’s and cross all the t’s, good old moldy mold will appear in your tank out of nowhere. 

This article will discuss the different kinds of mold that plague our tanks, a foolproof plan to get rid of them, and the possible culprits behind the mold growth. 

So, let’s get going! 

5 Types Of Molds That Plague Aquariums

Mold is basically a fungus made up of microscopic organisms and can come in various colors and shapes. For molds to grow and proliferate, they need a humid environment – and there’s no perfect place for this than our home aquariums! 

There are primarily 5 kinds of mold that grow in our tanks. They are: 

  • Green fungus
  • Black fungus 
  • Black beard algae 
  • Algae 
  • White fungus 
Green fungus 

Green fungus is often mistaken for algae. But before you know it, it will make a home in all parts of the tank and won’t even spare your fish’s body. The leading cause of green fungus is foul water parameters. 

Black fungus 

Black fungus manifests as unsightly small black patches. Its growth is often linked with heightened levels of phosphate in the tank. 

Black beard algae

Black beard algae is the fuzzy black mold hailing from the family of red algae that thrives in high-phosphate environments. Unfortunately, scrubbing it away is only a temporary solution – it will keep haunting you until you seek a permanent solution. 


You already know what causes algae in the tank – excess nutrients, foul water, and too much light. While algae itself isn’t as harmful as some listed here, it definitely makes the tank look unsightly!  

White fungus  

The white fungus first forms in decorations, plants, gravel, and rocks before breaking off the items and floating around the tank. This type of fungus is notorious for thriving on wounds and skin tissues. 

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How To Get Rid Of Mold In Fish Tanks?

Here’s your one-stop guide to removing mold in fish tanks. 

First, let’s begin with the list of tools you need:

  • Hot water 
  • Vinegar 
  • An aquarium siphon 
  • Fishnet 
  • A replacement filter cartridge 
  • Two large containers 
  • Toothbrush 
  • Sponge
  • Algae scraper 
  • Paper towel (chemical-free)
  • Water conditioner 
  • Water clarifier

Step 1: Clean Your Hands 

This might sound obvious, but I still didn’t want to skip this. The first step here is to clean your hands thoroughly. Otherwise, you’ll end up introducing new germs and pathogens into the tank that will backfire in unlikely ways. 

Clean your hands with soap and water so that you don’t transmit any contaminants into the tank that can potentially kill your fish. 

Step 2: Relocate The Fish 

Open and remove the mold-infested lid and keep it at a safe distance. Next, take a bucket and fill it with conditioned water. If you have an extra tank, even better! 

This is the water your fish will stay in until the tank’s all prepped up. Therefore, try to emulate the tank’s original water parameters as closely as possible. 

Using a fishnet, gently transfer all the fish to the bucket one by one. Make sure that you’re gentle with handling. If the fish get stressed, it will only make the entire procedure harder.

Step 3: Siphon The Water 

Now that the fish are out of the water, start removing the tank’s parts one by one. First, remove the gravel, plants, decors, and everything else. 

You can keep the pebbles and gravel in a colander. For decorations and plants, use a bucket or a bowl. 

Once everything is out of the tank, start siphoning the water with the help of the aquarium siphon. 

If the water isn’t entirely infested by mold, you can keep around 10% of water so that you don’t banish the good bacteria colony. Now, safely discard the siphoned water.

Step 4: Clean The Lid 

The lid is often the most densely infested part of the fish tank. To clean it, you will need to create a water and vinegar solution in a 1:1 ratio and pour it into a spray bottle. 

Next, start spraying the solution generously on every part of the lid. Let the mix sit for around 5 minutes and meticulously wash it using hot water. 

And by the way, it’d be better if you wore gloves during the entire process. 

Continue the exact steps of spraying the solution, letting it sit, and washing with hot water for a second or third time as needed. 

If the mold still hasn’t vanished, you might want to use a scrubber and some more elbow grease to get rid of it. 

Also, once the mold is gone, you have to wash the lid thoroughly and ensure no traces of vinegar are left in it. It can be dangerous to the fish. 

Step 5: Clean The Gravel 

To thoroughly clean the gravel, you need to soak them in a bleach and water solution for some time. An easy way is to plug the sink’s drain, pour gravel into it, and fill it with water. Next, add about ¼ cup of bleach. 

Leave the gravel to soak in the solution for about 15 minutes. And while the gravel is marinating, you can switch to cleaning plants.

Step 6: Clean The Plants and Decorations

Using a paper towel, wipe down all the plants and decorations. If it’s a natural plant, you need to be very delicate with the handling. If it’s a plastic plant, the mold will slide off the plastic and onto the paper towel without any problem. 

Repeat this step a couple of times using a new paper towel every time to not smear the mold around. 

If you don’t have paper towels handy, microfiber cloths will work great too. To ensure that there’s no mold left, use your fingers to push the paper towel into every nook and cranny of the plants and decors.

You should avoid using disinfecting agents to clean plants and decors, as even the slightest traces of chemicals can be toxic for your fish. 

Now, thoroughly clean the decors and plants using water. For real plants, you might want to stick to cold or lukewarm water. For the rest, hot water would be more effective. 

Lastly, if your decors are sturdy enough and microwave-safe, you can sterilize them by putting them inside a microwave and turning it on for about 20 seconds. 

Sterilization will further assure that your decors are clean and safe for your fish. 

Step 7: Rinse The Gravel 

Drain the sink and rinse the gravel under hot running water for at least 10 minutes. You really need to be meticulous with this step.

If there are any traces of bleach left behind, it can downright kill your fish. Stir and shake the gravel to ensure all bleach residues are gone. 

Step 8: Clean The Tank

Using an algae scraper, scrape off any mold present in the fish tank. Wipe and clean the tank’s insides but don’t use any cleaning agents to do so. 

Once you have scraped the prominent areas, use a toothbrush to scrub off the inner corners. Now, use chemical-free tissue paper to wipe away the scraped and brushed gunk from the tank. 

You can use vinegar to clean the outer surface of the tank. 

Rinse the tank thoroughly with hot water and keep siphoning it until the water runs clear. 

Step 9: Set Up The Tank 

First, replace your filter cartridge and rinse the filter with hot water to set up the tank. Next, slowly arrange the tank’s decors and plants back in their original places. 

Fill the tank with water and add a dechlorinating agent to eliminate chlorine and other harmful chemicals. You should also use a water clarifier to dissolve excess substances that encourage the proliferation of mold and other harmful organisms. 

Here’s an in-depth article on how to use a water conditioner in a fish tank. 

Related: Can Too Much Water Conditioner Kill Fish? It Can Bind Oxygen!

Step 10: Bring The Fish Back Home

Once the parameters become suitable for the fish, gently transfer the fish one by one into their newly cleaned and refurbished home. 

You can also add suckerfish and snails to the tank, which will help to keep mold and algae growth under control! 

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Why Is There Mold In My Fish Tank?

Moldy Tank
Credit: Damitr (CC License)

The most common reason behind mold in tanks is under maintenance – you either don’t remove the leftover in time, don’t perform enough water changes, keep the lights on for extended periods, and so on and so forth. Other times, it’s the municipal tap water’s fault – which contains higher phosphate concentrations – encouraging mold’s growth. 

Leftover Food 

The most pervasive reason behind mold in fish tanks is the pileup of leftover food. The food that your fish don’t eat starts to accumulate in the sides and the bottom of the tank. And before you know it, these gunk will start growing mold. 

Fish can thrive with minimal amounts of food. But all of us are guilty of overfeeding them and not removing the leftovers in time. This gives rise to phosphoric fumes, and these fumes cause mold to grow. 

When your fish inhales phosphoric fumes, it’s going to deteriorate their health seriously. 

Old Plants And Decors 

We often tend to overlook plants and decorations when cleaning the aquarium. But these unlikely suspects can very well be the reason behind the mold growth in fish tanks. 

For instance, if you keep live plants in the tank, you need to maintain, prune or replace them once every few weeks. That’s because when the plants start to die, the oxygenation process becomes very slow. 

And consequently, the tank’s environment reaches a stage where you will see brown or black mold growing on the corners and the tank’s surface. 

Cloudy Water 

Failing to perform routine water changes will increase the piling of excrement, gunk, food particles, and debris in the tank – making the water cloudy and dirty. 

Foul substances and imbalanced parameters lead to a spike in harmful bacteria and their byproducts that give the tank a cloudy appearance. 

And a cloudy tank is a breeding ground for mold.

Excessive Light 

Only a handful of aquatic plants need light for over 8 hours a day. And almost no aquarium plant falls into that category. Keeping the lights on for longer durations than required will fuel the mold’s growth.

And that’s because mold loves light – especially sunlight. So if your aquarium is kept in a place that receives direct sunlight, the environment becomes more than suitable for mold growth. 

Basically, when there’s a surplus of incoming light than what plants need, fungi and mold make an entrance. 

Nutritional Deficiency 

The tank’s nutritional requirements are a lesser-known reason behind unsightly mold growth. For example, a lack of enough carbon dioxide in the tank will fuel the growth of black beard algae – a type of black mold. 

The lack of enough CO2 in the tank means your plants will not be able to outcompete algae for nutrition due to their inefficient photosynthesis process. 

High Phosphorus Content In Tap Water 

In regions that still depend on old lead plumbing from the Victorian time, the water authorities have to use higher phosphate concentrations to prevent lead corrosion. And by now, you already know that phosphate is linked with mold growth. 

Therefore, you should always check the phosphate levels first if you see black molds developing in the tank. 

Here’s a link to the phosphate test kit from API I use:

Wrong Frequency Of Water Changes 

Overdoing and underdoing are both harmful when it comes to performing water changes. If you don’t perform enough water changes, it will quickly make the water foul and create a favorable condition for mold growth. You know the drill. 

However, changing water more than needed can be equally dangerous, if not more.

This way, you will be annihilating the good bacteria colonies that work to break down harmful compounds into harmless wastes that will be filtered. 

Frequently Asked Questions 

Credit: Alexander Davronov (CC License)

Can Fish Tank Mold Make You Sick?

Yes, fish tank mold can become dangerous and even make you sick if you allow it to grow to large volumes. The main concern with molds is that one can inhale mold spores into the lungs, where they will produce mycotoxins.

In turn, these mycotoxins will cause allergies. 

And that’s not all. Some other health issues often associated with mold are:

  • Sore throats 
  • Headaches
  • Eye infections 
  • Sinus complications
  • Toxic mold syndrome 
  • Pulmonary hemorrhage 
  • Asthma

Can Mold In Fish Tanks Kill Fish? 

Yes, most kinds of mold can effectively kill fish. They can seldom directly cause a fish’s demise, but they’ll make the water so toxic and inhospitable for the fish that it will slowly succumb to death. 

That being said, not all molds are downright harmful to the fish. For instance, black beard algae – often known as black mold – do not pose any danger to fish. 

As unsightly as it looks, it doesn’t release any harmful toxins into the water. But the same can’t be said for all types of molds. Therefore, it’s always best to err on the side of caution. 

Why Is There Mold In Fish Tank Filter?

Sometimes, the tank’s conditions can be perfectly healthy – and there still will be mold growing in the fish tank. The most likely scenario here would be that some food got trapped in the filter and went moldy. 

It’s not downright dangerous to your fish, but you should still thoroughly clean the filter and change the filter media if needed. 

How To Get Rid Of Black Mold In Fish Tanks?

9 out of 10 times, black mold present in fish tanks is black beard algae. And the most likely cause behind it is a high phosphate concentration in the tank.

To remove it, you’ll need to go through the 10 steps mentioned above, like scraping the glass, changing water, deep cleaning decors, and so on.

Simply scrubbing it away won’t work. 

Here’s an article where I have written about black mold and its eradication measures in detail. You might want to have a look! 

Related: Black Mold In Fish Tank? Here’s What You Should Do?

How To Get Rid Of White Mold In Fish Tanks?

White mold is fungus, and you should remove it as soon as you see it! The

10 steps that I shared above, such as deep cleaning the decors and gravel, changing water, and performing a water change, help get rid of white mold from the tank. 

Final Words: Mold In Fish Tank? 

So, as you can see, mold growth in tanks can happen for a myriad of reasons. Sometimes it’s because you slacked and didn’t remove leftover food in time; other times, it creeps on you alongside your new fish or plant. 

There are different kinds of molds, as we discussed above. And unfortunately, mold doesn’t just make the tank look unsightly but poses a serious health threat to you and your fish!  

The 10-step guide that we shared will help you sweep out the mold entirely and prevent it in the future. Just make sure you don’t skip any step. 

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