White Cotton Stuff in Fish Tank – Causes & Treatment

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Are you dealing with a white, slimy, cotton-like growth in the aquarium? If so, you should know this is a common occurrence in uncycled or unkept tanks. Its presence suggests there’s an unbalance in the tank’s ecosystem.

To clarify, that white cotton stuff in your tank is a species of fungus. So, it makes sense that it grows in high humidity and unsanitary conditions.

But don’t panic yet! As you’ll see, this issue is easily treatable. I’ll cover all the steps to get rid of this fungus in wide detail.

But first, let’s go over some of the root causes. This will help you better understand how and why this fungus thrives in your aquarium.

What Causes White Fungus in Fish Tank?

There are a few reasons why white fungus might appear. As you’ll see, the most common causes have to do with poor water quality.

But let’s take a close look at all of them, one by one:

– Accumulation of food and fish waste

Species of white fungus feed off decaying matter and thrive in unsanitary conditions. If you provide a constant source of food (in the form of food or fish waste), the fungus is going to thrive and spread like wildfire.

The most likely cause of waste is your overfeeding the fish. When you give your fish more food than they can eat, the excess will quickly accumulate and break down.

The decaying food will encourage the fungus to grow even more. On top of rotting food leftovers, overfeeding also leads to excessive fish waste.

Another cause of excessive waste is overcrowding. If you have a nano-tank or an otherwise overstocked tank, the bioload might be too much for the bacteria and filter to handle. Water quality is harder to maintain in such conditions.

You might not be overfeeding your fish per se. But the waste they produce might be too much for your tank size and water volume.

– Increased nitrate levels

Fish have a pretty good tolerance to nitrates. Levels of up to 40 ppm are perfectly safe for them. Although not ideal, some species might tolerate even higher concentrations, sometimes up to 80 ppm!

However, there’s a reason why the ideal nitrate levels are 20 ppm and lower.

Many parasitic species, white fungus included, thrive in nitrate-rich water. The nitrogen in the nitrates feeds and fuels the growth of fungus, algae, and even some strains of bacteria.

Besides, high nitrate concentrations come together with poor water quality.

– A spike in ammonia

The ideal ammonia level in a fish tank is 0.0 ppm. That means there’s no tolerable upper level of ammonia that’s safe for fish.

If you see a spike in ammonia, no matter how small, that’s a sign the bacteria in the tank can’t keep up with the filtration demands.

Increases in ammonia are also closely related to unsanitary conditions. Chances are if there’s ammonia in the water, there’s also excessive waste.

Just like nitrates, ammonia represents an important source of nutrition for white fungus.

The higher the concentration, the easier it is for the fungus to grow. If the white cotton stuff is widespread, you should test your water quality immediately.

Ammonia levels above 1.0 ppm can kill sensitive fish and otherwise sicken hardy species.

– Low oxygen in the water

Most species of mold grow in strictly aerobic conditions. This means they need oxygen to feed and develop. But the white fungus is a bit special.

It can grow in both aerobic and anaerobic conditions. Lower oxygen concentrations fuel its growth even more.

You might see a spike in white fungus growth when the oxygen is in the water tanks. This can happen due to a variety of reasons.

Some of these include overcrowding, stagnant water, poor filtration, algae overgrowth, excess waste, and high water temperature.

– Dirty substrate or decorations

Maybe your aquarium is otherwise clean. Your filter mechanically removes most of the waste and you don’t overfeed your fish.

Well, white fungus can find nutrients in other places too. Anywhere where there’s dirt, the fungus follows.

There are parts of the aquarium that you might not pay close attention to. This includes decorations like rocks and driftwood, narrow crevices like the inner corners of the tank, and also de underlayers of the substrate.

Notice where the fungus first starts growing. If the white cotton growth is attached to driftwood or rocks, this means there’s a lot of gunk for the fungus to feed on.

– You use chlorinated water

It’s a mistake we’ve all accidentally made at least once. Sometimes, you forget to condition your water, so you accidentally introduce chlorine into the aquarium.

This can be dangerous for fish. But equally important is the negative impact it has on water quality.

Chlorine kills bacteria. The nitrifying colonies you’ve worked hard to culture might have vanished.

This can lead to creeping ammonia levels and otherwise poor water quality. With the bad bacteria gone, the white fungus is free to feast on the waste by-products and take over the tank.

How to Get Rid of White Fungus in Fish Tank?

White fungus is a parasitic organism. It thrives in bad water quality. It also promotes poor water chemistry.

It’s dangerous for both the plants and fish in the aquarium. Luckily, there are a few things you can do to get rid of it.

Note that you’ll have to move your fish to another aquarium while following these steps:

– Clean aquarium décor and equipment

The first thing you want to do is eliminate the source of the issue. White fungus thrives in unsanitary conditions and feeds off debris and decaying matter.

Once you take these out, you’ll solve half the problem. You’ll have to remove everything from your tank for a thorough cleaning.

Rinsing the substrate, decorations, and plants under lukewarm water should be enough. Give the decorations a thorough scrub to clean all the crevices.

Don’t use any detergents. Avoid using chlorinated water when rinsing the substrate. This will kill the beneficial bacteria in the soil.

Next, take out the equipment. You’ll have to clean the filter too! But be very careful if you’re using a sponge or ceramic rings. These types of filter media can harbor beneficial bacteria.

You don’t want to accidentally kill these with soaps or chlorine. Otherwise, you’ll have to cycle your aquarium again! Just rinse the filter media with some clean aquarium water instead.

– Thoroughly clean the aquarium walls

Don’t forget the inside of the tank! Dirt can lurk anywhere the aquarium water is present. You’ll have to wipe down the inner walls of the aquarium.

Don’t forget the harder-to-reach areas like the inner corners!

It’s best to do this after removing the substrate and decorations. This will give you better access to the floor of the aquarium for a thorough cleaning.

You can use any clean towel or brush. Also, remember to collect and remove the floating bits of fungus that end up in the water.

– Do a large water change

The water should be clean, too. You’ll have to remove most of the aquarium water to get rid of all the debris that’s left.

Shoot for a 50-95% water change, depending on your aquarium size. Remember to closely monitor the water parameters after this.

You might notice some fluctuations in water temperature and hardness after such a large water change.

You might have to treat the water accordingly, depending on your fish’s ideal parameters. Apply water hardener or softener as required.

– Consider using anti-fungal medication

Your aquarium might look clean after all the scrubbing and water changes. But some tiny spores might still be hiding in plain sight. Try using some anti-fungal (also known as anti-mold) medication to be extra safe.

You can find fish and plant-safe medication in most pet stores, as well as on Amazon. The treatment will take anywhere between 24 hours to 5 days, depending on which medication you choose.

Methylene Blue is a highly effective and widely-available option. It treats mold, fish infections, as well as ammonia and nitrite poisoning.

It’s also plant-safe and very easy to dose. Just know that it will stain the water blue. Getting rid of the color will require multiple water changes.

– Increase water surface agitation

Stagnant water and low-oxygen conditions encourage white fungus growth. When the water is still, the exchange of gases slows down or stops completely. This means that new oxygen can’t enter the water, and the CO2 produced by the fish can’t travel back out.

Luckily, this issue is easy to fix. Consider adding an air stone to your aquarium, if you don’t have one already. This will create more movement and surface agitation.

This will break the surface tension, letting the water trap more oxygen and release carbon dioxide.

An air stone is all the more important if you have a small or overstocked aquarium. Fish consume lots of oxygen. You need to replenish the oxygen constantly to keep a positive balance.

– Bump up the temperature

White fungus thrives in a cold environment. You can kill or prevent this mold from growing by turning up the temperature.

Aim for temperatures above 80°F for extra protection. Of course, this method might not be suitable if you keep cold water fish like Goldfish, Minnows, Barbs, or Corys.

Also, know that higher temperatures can come with some side effects. Temperatures above 80-82°F may encourage higher appetite, energy levels, and breeding in certain fish species. Higher water temperature also holds less oxygen than cooler water.

– Use a biological filter

If you don’t have one already, it’s time to invest in a biological filter! This type of filter comes with porous materials that provide a large surface area for nitrifying bacteria to colonize. When using a biological filter, you can double the colony of beneficial bacteria in the tank.

If you already have a biological filter, you can also use concentrated nitrifying bacteria for extra help. You’ll find these as bottled products under different names.

They’re usually described as “nitrifiers” or “quick starters”. They’re used to speed up the nitrogen cycle, but can also be added to already cycled tanks.

Why are nitrifying bacteria so important? They keep the water clean and prevent ammonia and nitrite spikes! This creates a cascade of beneficial effects for your aquarium.

By properly managing waste by-products like ammonia, you also keep algae and fungus at bay. This also helps you maintain stable water pH and oxygen levels.

– Monitor water parameters and cleanliness

Prevention is key when it comes to parasitic fungus and algae. Just because you treated the problem, doesn’t mean it can’t return! To keep other future infestations at bay, remember to be on top of the maintenance schedule.

You’ll have to perform small weekly water changes. Replace up to 25% of the water each week. Also, don’t forget to siphon the substrate at least twice a month.

Finally, remember to do a thorough aquarium cleaning once per month. This includes wiping down the decorations, aquarium walls, and plant leaves.

Stick to this routine and you should avoid fluctuations in ammonia, nitrites, or pH. Just to be sure, remember to check the water parameters regularly.

A good place to start is testing the water every time you make a water change.

– Don’t overfeed the fish

Overfeeding is the number one cause of ammonia spikes. It creates a higher bioload than your filter and bacteria can handle.

Reducing the quantity or frequency of feeding will improve the water quality. It also makes cleaning the tank and managing the water parameters more easily.

How much or how often you feed your fish will depend on the species you have. But as a general rule of thumb, you should only feed your fish as much as they will eat in one or two minutes.

Most fish only require feeding once or twice per day. However, the fry will eat more often.

– Consider moving some of the fish

If your aquarium is overcrowded, you should rehome some of the fish. This will greatly reduce the bioload in the tank. You’ll get less fish waste and food leftovers stinking up the water. The pH, nitrate, and oxygen levels will also be more stable.

To figure out if your tank is overcrowded, you’ll have to consider the aquarium size and the number of fish you keep.

As a general rule, you should provide one to two gallons of water for each one inch of fish. So, a 3-inch fish will need at least 3-6 gallons worth of aquarium space.

Take the total water volume in your tank and divide it by two. That’s the total number of inches. Now, divide this number by the maximum size of your fish species.

That’s the total number of fish you should keep to avoid overcrowding.

Say you have a 40-gallon aquarium. Divided by two, that gives you a total of 20 inches. If you were to keep only Tetras, you’d divide that 20 by 1.5 inches (max Tetra adult size). This means you can keep roughly 13 Tetras in your tank.

Is White Fuzzy Fungus Dangerous to Fish?

White fungus is more than just an eyesore. Indeed, this parasitic fungus can be dangerous for your fish.

This strain of mold is an opportunistic feeder. It will spread and consume any resources available in the tank.

This means it can also infect your fish! It can grow in fish wounds, and may also cause eye and mouth infections. The white fuzzy fungus can also cause bladder and stomach issues in fish.

To make matters worse, fish might accidentally consume this fungus while feeding or nibbling on plants.

Besides the direct negative impact, white fungus can also cause issues indirectly. This fungus thrives in and promotes poor water quality.

Your fish can develop ammonia burns and other diseases even without being directly infected. The unsanitary conditions in the tank also predispose the fish to other bacterial and fungal infections.

Is White Fungus Dangerous to Plants?

White fungus might not directly harm your plants. But the plants and fungus have to compete for nutrients in the tank.

The plants need CO2 and nitrogen to grow. The fungus uses the same nutrients to feed and proliferate.

This might lead to stunted growth and deficiency symptoms in plants. Some of the symptoms you might notice include pale or yellow leaves, wilting, and leaves presenting spots and small holes.

Fungus might also grow directly on the plant leaves and roots, further impacting nutrient absorption.


White fungus frequently grows in uncycled tanks or in unsanitary conditions. This parasitic species thrives in water with high ammonia and nitrate levels.

It also grows best in poorly oxygenated water. All of these issues can be traced back to a few common causes.

You’re more likely to have poor water chemistry if your aquarium is unkept or overcrowded.

Infrequent water changes, dirty substrate and decorations, excessive feeding, and fish waste can all contribute to this.

Sometimes, the fungus can also grow if the nitrifying bacteria are missing. This is the case in uncycled tanks.

Sometimes, you might also accidentally kill the bacteria with chlorinated water or detergents when cleaning your tank.

Getting rid of the fungus requires reintroducing beneficial bacteria into the tank. You can either add a biological filter or even concentrated nitrifying bacteria.

Maintaining a regular cleaning schedule and avoiding overcrowding and overfeeding will also help.

Author Image Fabian
I’m Fabian, aquarium fish breeder and founder of this website. I’ve been keeping fish, since I was a kid. On this blog, I share a lot of information about the aquarium hobby and various fish species that I like. Please leave a comment if you have any question.
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