If you have noticed white, fuzzy stuff on your fish, you are probably dealing with either fungus or columnaris. They are often mistaken for one another, given their characterized fluffy, cotton-like patches on the fish’s skin or gills.
Unfortunately, no matter which one your fish is suffering from, fungus or columnaris, it needs to be treated as soon as possible.
Learning the difference between fungus Vs. columnaris will help you look for an underlying cause and choose the right treatment for your fishy friend – before it gets out of hand.
So, we’ve put together this handy guide to telling the two apart.
Characteristics: Fungus Vs. Columnaris
It’s important first to establish what defines a fungus versus columnaris. The two cause similar symptoms in affected fish, but they are two unrelated pathogens.
Understanding the characteristics of fungus (Saprolegnia spp.) and columnaris (Flavobacterium spp.) is essential for the correct diagnosis and treatment.
What Is Columnaris Disease?
Columnaris (sometimes called cottonmouth or cotton wool disease) is a common bacterial infection that can affect both aquarium and wild freshwater fish, particularly livebearers and catfish.
This disease is mainly caused by the Gram-negative bacterium Flavobacterium columnare (F. columnare). Fish of all ages are considered susceptible to this disease, but it is more commonly found in juveniles .
In addition, F. columnare is warmer water bacteria that thrives in waters warmer than 68° F (20° C) , even though it can survive in water as cool as 53.6° F (12 °C).
Beyond that, another important biochemical characteristic of F. columnare that aid in distinguishing it from fungus is it’s a strict Aerobe  that needs oxygen for growth and survival.
What Is Fungus Disease?
Fungus is an infection caused by fungus spores. These spores can be found in almost any water source, so it’s important to treat and quarantine fish as soon as possible if you suspect fungus might be the cause of your fish’s illness.
any species from the genus Saprolegnia are the causative agent of fungal infections. They are most commonly found in poorly maintained aquariums, which can rapidly lead to secondary bacterial infections.
Sometimes, the Saprolegnia fungus is also called a cotton mold or water mold, and it has been known to affect most freshwater fish.
Saprolegnia has the ability to survive in a wide temperature range, from 37°F to 91 °F (3 – 33 °C)  but is more common in cooler waters.
Unlike F. columnare is non-halophilic bacteria, meaning it does not live in saltwater, Saprolegnia can tolerate brackish water.
Due to their same opportunistic, strict aerobic nature, Saprolegnia and F. columnare cannot thrive in poor-quality water with low oxygen levels. As a result, they can often be present in well-circulated, oxygenated tanks. Adding aeration or increasing circulation will do nothing to prevent Columnaris and fungus other than to possibly help the fish fight these two diseases.
How to Identify a Fungus Vs. Columnaris
It can be hard to distinguish fungus from columnaris by checking the fish’s behavioral tendencies since they are too subtle to recognize. If you are concerned about a fish, check out the following identifiers instead.
Without a microscope, the grey, brown, or white hair-like structures of Saprolegnia (Fungus) are visible to the naked eye by using a magnifying glass.
However, you must access a microscope to examine the gliding, long rod-shaped F. columnare (columnaris) even though they often aggregate and form microscopic columns (hence the name).
Another big consideration when trying to distinguish between fungus and columnaris is that fungal patches are typically found only on the dead tissue of your fish, unlike Columnaris, which need to enter and live on the living tissue of the host [x]. The fungus also often can appear on your shrimp, decoration, plants, etc.
Columnaris infections are more likely to spread quickly, particularly in acute cases. Saprolegnia (Fungus), on the other hand, progresses relatively slowly, taking many days before harming or even killing fish.
Columnaris is an external bacterial infection that should be treated with antibiotics. Sometimes, the treatment of Fungus (Saprolegnia) is somewhat similar to that of Columnaris, as most Saprolegnia infections can promote secondary bacterial infections.
However, the mechanisms of antibiotics, no matter whether they are Gram-positive or Gram-negative, use target bacterial metabolism that fungi don’t have, meaning antibiotics have little effect or don’t work for fungal infections.
Several medications have proven to be quite effective in treating Columnaris, yet unfortunately, there is a great deal of inaccurate information out there.
Myth #1: Erythromycin (aka Maracyn)
Many online forums will recommend using Erythromycin or other members of the tetracycline class to treat columnaris and fungus. Unfortunately, most Tetracyclines are Gram-positive antibiotics, and F. columnare is a Gram-negative bacterium.
Author notes: Doxycycline is a broad-spectrum tetracycline, and it is the ONLY one that can be effective in treating true Columnaris.
Myth #2: Raising Your Water Temperature
Also, be careful with this advice. While it is true that raising the temperature can speed up the life cycle of certain bacteria, like Ichthyophthirius multifiliis(Ich), it does not work against F. columnare (Columnaris) because, as we mentioned earlier, it’s warmer water bacteria that thrive in temperatures above 68°F (20°C).
Best Course of Action
Once you get a correct diagnosis, here are the best treatments of Columnaris you can follow:
Step #1: Stress Reduction
Consider reducing or eliminating any primary stressors; this includes overcrowding, incompatible tank mates, and low water quality.
Step #2: Set up a Hospital Tank
Since the antibiotics will kill your nitrifying bacteria, setting up a hospital tank ready to quarantine any potentially sick fish is best.
Step #3: Salt Bath
Still remember that F. columnare (columnaris) is a non-halophilic bacterium? It means it’s highly sensitive to salt. So, aquarium salt is a great way to disinfect water contaminated by F. columnare.
You can either add aquarium salt directly to your main aquarium at a rate of 2 Tbsp per 5 Gallons of water or pour the salt directly into the hospital tank at a higher salt concentration (1 Tbsp per 3 Gallons of Water) and submerge the fish for 30 minutes.
Closely observe your fish during the salt bath. If you see any signs of distress, immediately remove them from the tank and move back to the main tank.
Step #4: Methylene blue (MB) Bath
As you probably already know, Columnaris can spread the gills and then enter the bloodstream without a few other symptoms. For this reason, a strong Methylene Blue Bath can be a great commercial solution.
Methylene blue is an antifungal medication in aquaculture and is commonly used to treat fungus infecting fish and its eggs, often combined with a salt bath.
Methylene Blue dosage: 1 Tbsp (5 ml) of 2.303% Methylene Blue per 10 gallons of water. Keep the fish in the solution for 3 to 5 days, and then increase the concentration if there’s no effect.
Step #5: Broad-spectrum Antibiotics
Remember, this strongest medication treatment for Columnaris only works as oral feeding, so carefully read the labels for information.
You must address the causes of fungus problems since it’s often found in fish with a simultaneous bacterial infection. If your fish has true fungal (Saprolegnia) infections, we recommend using Acriflavine (API Fungus Cure) for mild to moderate fungus infection and a combination of Malachite Green (or Methylene Blue) with Nitrofurazone (Furan-2) for serious cases.
Author notes: Methylene Blue is best used in a bath solution.
So, there you have it – the truth behind fungus Vs. columnaris treatment. Remember, fungus and columnaris are two different diseases, so you must use the right treatments for each.
It’s always a good idea to consult your veterinarian and diagnose correctly with a microscope.
If you have any experiences with fungus or columnaris, we’d love to hear from you. Share in the comments below!
- Columnaris Disease [Fisheries.org]
- Columnaris Disease [ScienceDirect]
- Declercq AM, Haesebrouck F, Van den Broeck W, Bossier P, Decostere A. Columnaris disease in fish: a review with emphasis on bacterium-host interactions. Vet Res. 2013 Apr 24;44(1):27. doi: 10.1186/1297-9716-44-27
- Saprolegnia [ScienceDirect]