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White Liquid Slime In Fish Tank: Causes & Treatment

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I remember how worried I was when I first came across a while liquid slimy substance on the driftwood in my aquarium.

I was sure it was something toxic that could harm my fish and destroy the water chemistry. Fortunately, it was nothing like that.

In this article, I will help you understand what stands behind this problem and how to fix it quickly. Let’s dive right into it.

What Is The White Liquid Slime In My Fish Tank?

These are two main reasons for a white slime to appear in your fish tank:

  • Biofilm (bacteria) overgrowth – The most common cause.
  • Alcaligenes faecalis – A specific type of rod-shaped bacteria.

Let’s see why each one is created, where it usually appears and what steps you should take to get rid of it:

1. Biofilm (Bacteria) Overgrowth – Most Likely

Your approach to biofilm will depend on the context. 

Some people perceive it as a harmless component that appears in most, if not all, aquariums. It is neither dangerous nor particularly difficult to control. 

On the other hand, you have this paper in the Journal of Fish Diseases (Hector A. Levipan, Diana Tapia-Cammas, Rute Irgang) that highlighted the destructive consequences of tenacibaculosis on fish.[1]

According to the paper, it may cause severe tail rot, gross lesions, frayed fins, etc. The paper analyses biofilm formation, placing special emphasis on Tenacibaculum maritimum and other related bacteria.

You also have this paper in Applied And Environmental Microbiology (Wenlong Cai, Covadonga R. Arias, Leonardo De La Fuente) that blames biofilm for contagions in fish farms.[2]

The term ‘Biofilm’ is quite broad. You must emphasize the context whenever you consult an aquaculture expert about the biofilm outbreak in your aquarium.

In most cases, biofilm overgrowth is not harmful to fish. The articles mentioned above are far from the average scenario.

  • What Is Biofilm?

Pramod Kumar Pandey, Kunadan Kumar, and Vivekanand Bharti (African Journal Of Microbiology Research) define ‘Biofilm’ as a collection of microbial cells encased in polysaccharide material.[3]

Simply put, it is a mass of various bacteria that uses a clear slime to grip hard surfaces. 

The term ‘Biofilm’ encompasses algae, fungi, diatoms, and various microscopic organisms.

Aquarists tolerate the slimy substance because certain aquatic creatures eat it, the most prominent being shrimp. 

In fact, some people will supplement a shrimp’s diet by deliberately encouraging biofilm growth. 

But you must apply caution. First of all, excess biofilm signifies poor quality conditions.

Your attempts to bolster biofilm growth in the aquatic environment may destroy the tank’s chemical equilibrium by boosting ammonia, nitrite, and nitrate concentrations.

Additionally, a heavy biofilm infestation can increase CO levels while reducing the oxygen content in the aquarium.

Pro tip: If you’re new to this hobby, I highly recommend you check out my complete guides on ammonia and nitrate. This will save you a lot of trouble down the line.

  • What Causes Biofilm?
  1. Decomposing leftovers.
  2. Decomposing waste.
  3. The substrate is saturated with hydrogenous waste. 
  4. Dead and decomposing plants and animals.
  5. High ammonia and nitrite levels.

Poorly maintained tanks create a conducive environment for biofilms to thrive. But you can boost the growth of these organisms without compromising your aquarium’s health. 

For example, this can be done by feeding your fish and shrimp powdered foods (they allow nutrients to spread evenly). 

You may also unintentionally introduce driftwood that provides a suitable surface for biofilm to grow. This is also true for large leaves.

  • Where Is Biofilm Usually Found?

Biofilm generates a sticky slime that allows the microscopic organisms to attach to hard surfaces in wet environments, such as rocks, driftwood, leaves, tank walls, etc. 

The biofilm can even form on the water’s surface. You can boost biofilm growth by adding leaves, decorations, and other hard surfaces.

  • How To Identify Biofilm?

Look for a white, slimy substance. When it floats on the water’s surface, the biofilm looks like a thin oily layer. 

Unlike algae, these organisms don’t need light to grow. 

They may go unnoticed until they become thick enough to form a cloudy jelly-like substance spreading across the tank’s hard surfaces. 

Pro tip: Some aquarists confuse biofilm with snail eggs, as they both look like jelly. Here is an article where I talked all about it.

  • How To Treat Biofilm?

Your first and most obvious option is to do nothing. This substance will usually disappear on its own, without any direct intervention.

Every aquarium has biofilm. But in a well-maintained tank, the organisms are too few to stand out. 

The biofilm population may spike during the initial stages of your tank’s life cycle. 

But over time, once cycling is complete, and as conditions in the aquarium stabilize, the biofilm will disappear.

However, there are steps you can take to speed up this process:

  • Identify the infested surfaces, and scrub the biofilm off with a soft brush.
  • Clean the infested surfaces with a vinegar solution and rinse thoroughly afterward.
  • Add some shrimp to your tank. They eat the organisms.
  • Perform regular water changes (about 15 percent each week).
  • Use water conditioners to neutralize the ammonia and nitrites (if present). You can find more information about it here.
  • Avoid overfeeding – feed your fish the amount they can finish within two to three minutes.
  • Increase surface agitation to lower CO. You can easily do that with an airstone. I personally got the Hygger Aquarium Air Stone (link to Amazon).

You can experiment with disinfectants, but they don’t always work. 

According to a paper in Aquaculture Journal looking at biofilm bacterial communities in Mediterranean recirculation aquaculture systems, biofilms become resistant to disinfectants once they form.[4]

2. Alcaligenes Faecalis – Less Likely

Aquarists usually blame white liquid slime on bacterial blooms. But newcomers often struggle to identify the bacterial species at play. 

  • What Is Alcaligenes Faecalis?

If you typically associate white liquid slime with nitrifying bacteria, you’re not the only one. But this association is wrong. 

Nitrifying bacteria appear during the cycling process. They turn toxic ammonia into relatively harmless nitrates.

However, nitrifying bacteria will contribute to the water’s milky or cloudy texture. It doesn’t produce white slime. 

Another frequently cited culprit is cyanobacteria. But cyanobacterium is a form of algae. 

Even when it contributes to the white slimy organisms in the tank, the substances are normally fuzzy and reminiscent of algae.

Dinoflagellates are another common consideration. These unicellular protists have gold and brown coverings.[5]

Under the right lighting conditions, you may confuse them for white slimy organisms. However, a closer inspection will reveal bubbly snot that doesn’t fit your expectations.

This leaves you with Alcaligenes faecalis, a bacteria that produces white slime. The slime may take on yellow and brown hues over time. 

  • What Causes Alcaligenes Faecalis?
  1. High concentrations of phthalates.
  2. Alcohol.
  3. High levels of VOCs (Volatile Organic Compounds).
  • Where Are Alcaligenes Faecalis Usually Found?

If it goes unchecked, the bacteria will cover every object in the tank, including the plants and decorations. 

As an aerobic heterotroph, the organism will consume the oxygen in the water, suffocating the fish. The coral may die because of the bacteria’s acidic composition.

  • How To Identify Alcaligenes Faecalis?

The bacteria produces a white, slimy ooze that continues to grow until it covers everything. 

A microscopic analysis will reveal rod-shaped bacteria.[6] You may collect a sample and send it to a lab.

  • How To Treat Alcaligenes Faecalis?
  1. Identify and eliminate the factors responsible for the VOCs. That includes air deodorizers and any sprays that add toxins to the water.
  2. Acquire heterotrophic bacteria from your local retailer or any experienced aquarists you know. Heterotrophic bacteria will consume the same food Alcaligenes faecalis uses. In the process, they will break down the slime.
  3. Perform extensive water changes.
  4. Disinfectants like hydrogen peroxide will help. Pay attention to the quantities. Some species (such as corals) may respond negatively to high concentrations of disinfectants.
  5. Ask your local vet to recommend commercial products that remedy Alcaligenes faecalis infestations. A vet is essential because they take the species in your aquarium into account before recommending a suitable product.

Should I Be Worried About The White Slimy Stuff?

If this is your first time encountering this slimy thing, I recommend you wait and see if it goes away on its own.

In most cases, the problem will be resolved without any intervention. Don’t worry too much about it. It will not harm your fish population.

Also, there is absolutely no need to diagnose the material and tell exactly what it is – a biofilm or Alcaligenes faecalis.

It’s also quite challenging. Without a microscope, it’s impossible to tell the two apart. Both look like thin liquid slime covering various objects in your tank.

It might be worth a bit just to check the water parameters, including the ammonia, nitrite, and nitrate, just to see that everything is fine in this section.

Unlike biofilm or bacterial overgrowth, these toxins can negatively affect your fish. It could be that the substance is just a symptom of a more serious problem.

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If you found a white liquid slimy substance in your fish tank, it is probably biofilm. You usually see this on plants or driftwood as they make excellent growing mediums.

Fortunately, you don’t have to do anything to make it go away. It usually disappears without any direct intervention from your end. It won’t harm the fish either.

However, it is worth checking the water parameters. In many cases, biofilm overgrowth is a symptom of an ammonia spike.