Goldfish Fungus: Cotton Wool Disease vs. Columnaris

What is the white cotton-wool fluff on my goldfish?

Those fuzzy white or grey tufts are almost always one of two look-alike problems: a true water mold called Saprolegnia (classic "cotton wool" fungus), or a bacterial infection called columnaris that only looks fungal. Which one you have changes the treatment, so notice where the growth sits and what else is wrong in the tank.

True fungus (Saprolegnia) usually appears as white-to-brown, cotton-like tufts on the skin, fins, or gills — often right on a wound or a spot where scales have been lifted or lost [3]. A growth on the mouth as a grey-white or yellowish film, or a pale band across the body, is more suspicious for columnaris — which is a bacterium, not a fungus [4][5].

Is goldfish fungus actually a fungus — or is it columnaris?

This is the single most important and most-confused point, so get it straight before you medicate.

• True fungus is Saprolegnia, a water mold. Technically it is not even a true fungus — it is an oomycete, a fungus-like protist rather than a real fungus [1]. It produces the classic cotton-like growth on skin and gills [3].
• "Mouth fungus" / "cotton wool disease" of the mouth is very often not a fungus at all. It is usually columnaris, caused by the bacterium Flavobacterium columnare (older names include Flexibacter columnaris) [4]. As one aquatic-veterinary practice puts it, it's "not a fungus, it's a bacteria that looks like a fungus" [5].

Why does this matter? A mold and a bacterium respond to different medicines. Reaching for an antifungal when the problem is columnaris wastes time while a fast-moving bacterial infection spreads — and veterinary guidance is explicit that pinning down the organism is what leads to the right, targeted treatment [2].

What causes fungus on goldfish?

Here is the reassuring but important truth: fungus is almost never the first thing that went wrong. Water molds are secondary invaders that move in after an injury, another infection, or an environmental insult such as poor water quality or low water temperature [1]. Most live harmlessly in the water and only become a problem once the fish's skin is already damaged by abrasion or another pathogen [3].

So when you see fungus, the real question is: what stressed or injured this fish? Common culprits include:

• A wound, lost scales, rough handling, or netting damage [3].
• Poor water quality — ammonia and nitrite are toxic wastes, and high ammonia and high nitrite are known to invite disease [4].
• Wrong or swinging temperature. Cold water actually favors the mold, because chilly fish repair tissue and mount immune defenses more slowly [3].

Columnaris follows the same logic: it flares after environmental stress — low oxygen, high ammonia, high nitrite, high temperatures, rough handling, mechanical injury, and crowding are all on the list — and once established it can spread quickly and cause high mortality [4]. In farmed channel catfish, columnaris commonly strikes when water is 25–32°C (77–90°F) [4]; the takeaway for a cold-water goldfish is that unusually warm or fluctuating temperatures raise the risk.

What water conditions do goldfish need?

Goldfish are cold-water fish. Keep them at 65–75°F (about 18–24°C) with a pH of 6.5–7.5; they thrive in cooler water and are not suited to a heated tropical tank [6]. Do not borrow numbers from tropical species like bettas — a goldfish kept betta-warm is a stressed goldfish.

Your first response to any fuzz is to test and fix the water, not to dump in medication:

• Test ammonia, nitrite, and nitrate. High ammonia and high nitrite are known stressors that invite disease such as columnaris [4], so the practical goal is to keep both undetectable.
• Check the temperature and keep it stable. Steady, cool water helps a goldfish's immune system do its job; both cold snaps (which favor mold) and heat spikes (which favor columnaris) tip the balance toward infection [3][4].

How is goldfish fungus treated?

The honest answer splits by which organism you have.

Foundation (for both): correct the water quality and remove the underlying stressor. For external Saprolegnia, veterinarians note this often needs environmental and handling changes in addition to medication [2].

For true fungus (Saprolegnia): aquarists commonly use aquarium salt and over-the-counter antifungal remedies. That is common hobbyist practice, but I could not find a goldfish-specific salt concentration in an authoritative veterinary source — so treat any bottle-label number with caution and let an aquatic vet set the dose. In hatcheries, external mold is cleared with one-hour formalin drips [3]; commercial formalin and hydrogen-peroxide products are FDA-approved against fungus on fish eggs, with other uses requiring veterinary oversight [2].

For columnaris (bacterial): an antifungal will not help — this needs an antibacterial approach. In aquaculture, columnaris is managed by correcting water quality plus antibacterials such as potassium permanganate in the water and oxytetracycline in medicated feed [4]. A fish veterinarian notes that serious cases need robust, effective antibiotics, and over-the-counter products often do not qualify [5].

Two cautions: those aquaculture figures are pond doses for farmed food-fish, not aquarium recipes — do not copy them into your tank. And because fungus and columnaris need opposite treatments, guessing wrong costs time you may not have.

If your goldfish's spots look more like white grains than cotton tufts, that points toward ich (a parasite) rather than fungus — a different problem with a different fix.

Can I prevent goldfish fungus?

Mostly yes — prevention is really stress-reduction:

• Keep the tank clean and cycled — high ammonia and nitrite are disease-inviting stressors, so aim to keep them undetectable [4].
• Hold a steady, cool temperature in the goldfish range [6].
• Avoid injuries: gentle handling, no sharp décor, and no fin-nipping tankmates [3].
• Quarantine new fish before adding them, so you do not import columnaris or a parasite that opens the door to fungus.

Fungus on a fin can look a lot like fin rot in a betta — both are usually secondary infections riding on poor water quality. And note that swelling with raised, pinecone-like scales is a different picture called dropsy; see goldfish dropsy for that one.

When to See a Vet

Reach out to an aquatic veterinarian promptly if:

• The growth is on or around the mouth, or you see a pale saddle-like band — the columnaris picture, which can spread fast and kill quickly [4].
• The fuzz is spreading rapidly, covers the gills, or the fish is gasping, listless, or off its food — extensive or gill infection can be fatal from fluid imbalance and shock [3].
• Fish are dying, or several are affected at once [4].
• The growth does not improve after you have corrected the water and temperature.

Frequently Asked Questions

Is goldfish fungus contagious to me or my other fish?

There are no human health concerns associated with Saprolegnia, according to the Alaska Department of Fish and Game. It can affect other fish, because the mold spreads through the water — but it mainly takes hold on fish that are already injured or stressed, so healthy, unstressed tankmates are at lower risk.

How can I tell true fungus from columnaris at home?

Often you cannot be certain by eye. Per the SRAC columnaris fact sheet, columnaris tends to show as a yellowish-brown growth in and around the mouth, or a pale "saddleback" band, while true fungus (Saprolegnia) is usually cottony tufts on the skin, fins, or a wound. California Fish Veterinary Services stresses that columnaris is a bacterium that only looks like a fungus, and a definitive answer needs a microscope — which is why a fish vet is so valuable when the mouth is involved.

Will aquarium salt cure my goldfish's fungus?

Aquarium salt and over-the-counter antifungal remedies are common aquarist practices for true fungus, but I could not find a goldfish-specific salt concentration in an authoritative veterinary source, so let a fish vet set the dose. Just as importantly, salt and antifungals will not fix columnaris, which is bacterial and needs an antibacterial approach — a point made by both the SRAC columnaris fact sheet and California Fish Veterinary Services.

What temperature should my goldfish tank be?

PetMD's goldfish care sheet lists 65–75°F with a pH of 6.5–7.5 and notes goldfish are cold-water fish not suited to tropical tanks. Temperature cuts both ways: cold or fluctuating water favors the mold (per the Alaska Department of Fish and Game), while very warm water favors columnaris (per the SRAC fact sheet), so steady, cool water is safest.

Can goldfish fungus actually kill my fish?

Yes. The Alaska Department of Fish and Game notes that when external Saprolegnia is extensive or involves the gills, death is likely from fluid imbalance and circulatory shock. And the SRAC columnaris fact sheet warns that once established, columnaris can spread quickly and cause high mortality — which is why the mouth/columnaris picture is treated as urgent.

Does finding fungus mean I did something wrong?

Not exactly, but it is a signal to investigate. The Merck Veterinary Manual describes these aquatic fungi as secondary invaders that follow injury, another infection, or environmental insults like poor water quality or low temperature — so finding fungus is really a prompt to find and fix the underlying stressor.

References

1. Merck Veterinary Manual (Professional): Mycotic Diseases of Fish. https://www.merckvetmanual.com/exotic-and-laboratory-animals/aquarium-fish/mycotic-diseases-of-fish
2. Merck Veterinary Manual (Professional): Mycotic Diseases in Aquaculture. https://www.merckvetmanual.com/exotic-and-laboratory-animals/aquaculture/mycotic-diseases-in-aquaculture
3. Alaska Department of Fish and Game: Saprolegniasis – Cotton Wool Disease. https://www.adfg.alaska.gov/static/species/disease/pdfs/fishdiseases/saprolegniasis.pdf
4. Southern Regional Aquaculture Center (SRAC) No. 479: Columnaris Disease — A Bacterial Infection Caused by Flavobacterium columnare. https://aquaculture.mgcafe.uky.edu/sites/aquaculture.ca.uky.edu/files/srac_479_columnaris_disease_-_a_bacterial_infection_caused_by_flavobacteriun_columnare.pdf
5. California Fish Veterinary Services: Cotton Wool Disease aka Columnaris in Fish. https://cafishvet.com/fish-health-disease/cotton-wool-disease-aka-columnaris-in-fish/
6. PetMD: Goldfish Care Sheet. https://www.petmd.com/fish/goldfish-care-sheet

This is general information, not a diagnosis — talk to your veterinarian about your pet.