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White Cotton-like Fuzz In Fish Tanks: Causes & Treatment

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A few months ago, a friend of mine woke up to find his fish tank covered in a fuzzy white cotton-like substance. We both had no idea what it was.

As time went by, we dug deeper and deeper into this and found several possible causes for this problem.

To save you a lot of time and trouble, I decided to write this article, where I collect everything you need to know in one place.

What Is The Cotton-like Fuzz In My Fish tank?

The white cotton-like fuzz in fish tanks could be caused by fungus or white hair algae, but it could also be Saprolegnia or Columnaris, which are less common.

If you found a cotton-like fuzz in your fish tank, this is what you should know:

1. Fungus (Very Likely)

People use the terms fungus and mold interchangeably. ‘Water Mold’ is a broad term that encompasses over 200 species. 

But many people classify water mold as a category of fungus. Your approach to this organism won’t change regardless of what you call it.

The factors distinguishing ‘White Fungus’ from ‘White Mold’ don’t matter to the average aquarist. 

But if you have fuzzy white substances in the tank reminiscent of cotton, you are more likely to call them white mold.

The fuzzy white tufts in your tank might be fungus filaments that appear when organic matter decays. They thrive in environments with large quantities of waste.

2. Algae (Pretty Likely)

Like ‘Fungus’ and ‘Mold,’ people use the terms ‘Algae’ and ‘Fungus’ interchangeably in relation to the white substances they’ve observed in the water. 

The confusion makes sense because white algae have a fuzzy appearance and many aquarists use terms like ‘Cotton Growth’ to describe the organism.

But with algae, a closer inspection will reveal a slick film that is more stringy and not quite as fluffy as water mold. 

If you have staghorn algae, it won’t spread as rapidly as white fungus despite the many physical similarities between the two entities.

Coralline algae in saltwater aquariums can turn white due to exposure to the wrong pH and temperature.

But coralline algae are a crusty coating on live rock. You would never confuse them for white mold.

3. Saprolegnia (Not Likely)

White mold (or white fungus) is not a threat to your fish. You can ignore these organisms if your tank doesn’t have fish. 

Their presence is only problematic in a populated aquarium because it signifies poor-quality water. 

But what about the fuzzy patches you see on fish? Are they a cause for concern?

A study in Aquaculture and Fisheries (Xianle Yang, Siya Liu, Mao Lin, Renjian Ou, Pengpeng Song, Jiming Ruan, Kun Hu) analyzed Saprolegnia strains among freshwater fish in Southern China.[1]

They described Saprolegnia as a filamentous fungal infection responsible for severe losses in fish farms. 

In other words, this particular strain of white mold should concern you. Look for fuzzy white, gray, green, red, or brown patches on the fish.

While healthy fish can survive in tanks with Saprolegnia, factors such as poor water quality and the wrong parameters can weaken a fish’s immunity. 

That will make the creature vulnerable to the destructive consequences associated with Saprolegnia. Ask a vet to recommend medicated food. 

But they should only make this recommendation after taking a sample of the fish’s mucus to identify the pathogen. This will confirm your diagnosis. 

4. Columnaris (Not Likely)

Columnaris is not Saprolegnia. But amateur aquarists always confuse the two because columnaris can produce white, raised patches on the sick fish. 

This is why aquarists call columnaris ‘Cotton Wool Disease.’[2] But columnaris is not a fungal infection. It originates from bacteria. 

Other symptoms include lethargy, loss of appetite, and erratic swimming. Don’t blame these symptoms on white mold or fungus. 

Once again, you should consult a vet. They can provide clarification by sedating the fish and performing a small biopsy to prove that your fish has Columnaris as opposed to fungal infections like Saprolegnia. 

Once the vet identifies the strain, they can recommend effective treatments. The expert may recommend euthanasia if the case is severe. 

How Did My Fish Tank Get This Substance?

Saprolegnia, fungus, columnaris, and algae in aquariums typically spread because you used the following factors to create a conducive environment for the organism:

1. During Cycling

The fuzzy white stuff can appear during the cycling process because of the high concentrations of ammonia and nitrite. 

You introduce ammonia and nitrites by adding fish or waste, a tactic that benefits the substance because it encourages vital nutrients to proliferate. 

The objective of the cycling process is to nurture nitrifying bacteria, and in some situations, fungi and algae may appear to outcompete the beneficial bacteria. 

But eventually, the substance will fade as the bacteria multiply. You don’t expect white fungi or algae to persist in a fully cycled aquarium. 

2. Plants & Ornaments

Plants and ornaments play two significant roles in this situation.[3]

First, some decorations are already infested with white mold. Amateurs may ignore these fuzzy organisms because they don’t know any better. 

Therefore, these objects will introduce white fungus to the tank once you add them to the water. Secondly, fungi won’t thrive in a well-maintained tank. 

But you can undo the results of your strict maintenance routine by adding plants and decorations with decomposing organic matter.

Fungi will grow on these objects because they are rich in nutrients. But the fungi will eventually deplete their newfound food. At that point, they will fade.

3. Fish Waste & Overfeeding

Waste and leftovers will accumulate in a neglected tank, generating nutrients that encourage rapid fungi growth.

You can exacerbate this issue by overfeeding the fish. Overfeeding generates more fish waste and leftovers. The fungus will grow all over this decomposing waste. 

4. Elevated Toxins (Nitrogenous Compounds)

Fungi and algae will also flourish in the presence of nitrogenous compounds, including ammonia, nitrite, and even nitrate.

People associate high ammonia, nitrite, and nitrate levels with rotting fish waste and leftovers.

But you can introduce these toxins to your tank during a water change if your supply is contaminated.

5. Poor Circulation

Besides creating an oxygen deficiency, stagnant water allows fungi and algae to thrive. Weak filters create dead zones in the aquarium.

Also, tanks without aeration will suffer from carbon dioxide accumulation. 

And when you mix it with excessive exposure to light, you almost guarantee algae and mold overgrowth.

Where Are Fungus & Algae Usually Found?

Mold and algae will grow on every surface in the tank, including rocks, filters, hoses, ornaments, and more.

But again, white fungi or algae are not a cause for concern unless you have fish.

Fungus vs. White Algae: How Do I Tell The Difference?

Because mold is a type of fungus, it has many features people associate with fungi, including a fuzzy, fluffy, cottony appearance.

But it is less stringy, as opposed to white algae. The mold will thrive in environments with sufficient nutrients, such as phosphates and nitrates.

How Do I Remove It?

If you want to get rid of the white, cotton-like fuzz in your tank, it doesn’t really matter if it’s mold or algae.

Since both thrive in similar environments, the steps you need to take in both cases are the same:

  1. Remove the fuzzy growths manually. Be sure to wear gloves while doing so.
  2. Perform a partial water change. Depending on your fish’s health and the infestation’s severity, you can also do a 95 percent water change.
  3. Remove dead organic matter before it rots. Anything you can catch with your eyes must be removed, including dead fish.
  4. Install pump and air stones to improve aeration.
  5. Don’t overfeed your fish. Use an automatic feeder where necessary. Fish should only eat twice a day for a few minutes at a time.
  6. Use a vacuum to remove waste from the substrate. I personally recommend the Hygger Aquarium Gravel Cleaner (link to Amazon).
  7. Add fish that eat white fungi and algae, such as plecos, mollies, siamese algae, etc. For your convenience, here are my 32 best and worst algae eaters.

If you haven’t already, I highly recommend testing your water for ammonia, nitrite, and nitrate at least once a week.

I personally do this with the API FRESHWATER MASTER TEST KIT (link to Amazon), mainly because I’ve found it to be the most accurate.

  • For a better visual understanding of the steps listed above, here is an excellent YouTube video showing how to get rid of the white stuff:

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The white fuzz in your fish tank is probably fungus or algae. Fortunately, if you wish to get rid of it, it doesn’t matter which one it is.

In both cases, the material can be removed manually, at least some of it. The next step will be to do a partial water change and test for nitrogenous toxins.

Regular maintenance is also essential, as it prevents rotten residue from contaminating your water. Do not skip vacuuming the substrate.