Betta Fish Fungus: White Fuzz, Cotton Mouth, and How to Treat It

What Is the Cotton-Wool Fuzz on My Betta?

White or grey cotton-wool fuzz on a betta is usually one of two very different problems: a true water-mold "fungus" called Saprolegnia that settles on already-damaged skin, or columnaris — a bacterium that only looks fungal and needs a different medicine. Telling them apart, and fixing the water and temperature that let it start, matters more than any bottle off the fish-store shelf.

What does betta fungus look like — and what else could it be?

True fungus shows up as grayish-white, cotton-like tufts on the body, fins, eyes, or an old wound — not tidy dots or ragged fin edges. Water-mold (Saprolegnia) infection shows as greyish-white, cotton-like growths on the skin, gills, eyes, or fins. On a betta it usually looks like a fuzzy grey-white patch, often sitting on a spot that was already injured or frayed.

Two look-alikes are not fungus at all. Ich, or white spot disease, is a ciliated parasite that appears as discrete white dots, like grains of salt [4] — dots, not fuzz. Fin rot is bacterial erosion that leaves fins ragged or receding rather than cottony (betta fin rot); a betta can have both at once, because a frayed edge gives the water mold a foothold. A cottony patch on or around the mouth is a special case, covered below.

Why did my betta get fungus?

Water-mold fungus is almost always secondary: it colonizes a betta that is already weakened, so the real question is what stressed the fish first. The Merck Veterinary Manual describes aquatic fungi as secondary tissue invaders that follow traumatic injuries, infectious agents, or environmental insults such as poor water quality or low water temperatures [1] — they take hold in damaged or stressed tissue, not a healthy fish.

Temperature is the biggest betta-specific trigger. Bettas are tropical: a fish veterinarian's care guide recommends "78-82F (25-28C)" [7], and the University of Illinois College of Veterinary Medicine gives "76°F to 81°F," noting "an in-tank water heater will be needed" because most homes run cooler [6]. That same Merck page lists low water temperature among the environmental insults that predispose fish to these infections [1] — so a betta in an unheated bowl chills, the classic setup for fungus.

Water quality is the other driver. In a small, unfiltered bowl, ammonia and nitrite from waste burn the skin and gills; PetMD's vet-reviewed care sheet advises testing weekly for ammonia, nitrite, nitrate, and pH, keeping pH around 6.5–7.8 [8]. The 2013 Veterinary Research review even found high nitrite and organic load increase how readily the columnaris bacterium attaches to fish [5]. So before treating, hunt down the stressor: a cold bowl, dirty water, a nipped fin, or a recent move.

Is it true fungus or columnaris — and why does that change treatment?

If the fuzz is on the mouth or spreading fast, suspect columnaris — a bacterium, not a fungus — because in the warm water bettas need, it can turn lethal within a day or two.

Saprolegnia isn't technically a fungus at all; Merck notes that water molds such as Saprolegnia "are not true fungi but members of the Oomycota" [9], though it still responds to the antifungal-style treatments below. Columnaris is different: caused by Flavobacterium columnare, "gram-negative, rod or filamentous bacteria" per Merck [2] — a true bacterium. Its cottony look in tropical fish is exactly why, the 2013 review explains, the sign "led to the disease being termed 'cotton wool disease' or 'mouth fungus'" [5]; it can also form a saddle-shaped patch over the back or erode the gills and mouth [5].

Here is the betta-specific twist: Merck notes columnaris "is most common in warmwater species of fish" [2], and warmth makes it explosive. In salmon and trout studies summarized by the 2013 review, no fish died of columnaris at 9.4°C, but at 20.5°C almost all did [5]. Bettas are kept even warmer, at 78–82°F, about 25–28°C [7] — so the heat that keeps a betta healthy also supercharges this bacterium. That is the opposite of a cold-water goldfish, where chilling is the bigger driver (see goldfish fungus).

Why does it matter? Columnaris needs an antibacterial, not an antifungal. Merck says early cases may respond to potassium permanganate or hydrogen peroxide, but once the disease "becomes chronic, it may have become systemic, in which case treatment with florfenicol or oxytetracycline is recommended" [2] — prescription antibiotics dosed by a vet. Reaching for a fungus cure while columnaris races ahead wastes the little time you have; Merck warns its mortality "can be acute, and by the time the condition is seen it may be difficult to control" [3].

How do you treat betta fungus?

Start with the water and the stressor, because clean, correctly-warm water is what actually clears most fungus — then add a treatment matched to whether it is water-mold fungus or columnaris.

First, fix the environment: test and correct ammonia, nitrite, and nitrate [8]; do a partial water change with dechlorinated water; and add a heater to bring the tank into the betta's tropical range [6]. The foundation of treatment is cleaning up the water and correcting temperature; Merck notes that specific commercial formalin and hydrogen peroxide products are FDA-approved against fungus on fish eggs, and that use of these products on other life stages is extra-label and requires veterinary oversight [9]. The cleanup does most of the work.

For water-mold fungus (Saprolegnia), the chemical options a vet may reach for include formalin or hydrogen peroxide, which Merck describes as FDA-approved against fungus on fish eggs and extra-label (needing veterinary oversight) for other life stages [9] — potent chemicals that are easy to overdose in a small tank, so an aquatic vet should set the concentration. Aquarium salt and over-the-counter "anti-fungus" bottles are popular hobbyist support, but veterinary dosing for bettas is thin; treat any salt or med dose from a forum as hobbyist practice, not a proven prescription, and ask a fish vet before dosing.

For columnaris (the mouth or fast-spreading picture), the medicine is antibacterial: early cases may respond to potassium permanganate, while established cases need prescription antibiotics such as florfenicol or oxytetracycline, dosed by a vet [2]. An antifungal will not fix it. If your betta is also refusing food or floating oddly, rule out other problems (betta not eating, betta swim bladder).

How do I prevent betta fungus from coming back?

Fungus is really a symptom of husbandry, so prevention is a heated, filtered, cycled tank kept clean and stable. Give your betta a heater and a steady temperature in range — 78–82°F per a fish vet [7], or 76–81°F per PetMD and the University of Illinois vet college [8][6]. As Illinois puts it, bettas "should not live in bowls" and do best "in a 5-gallon glass or plastic tank or larger" with gentle, low-flow filtration [6].

Keep water quality high by cycling the tank, testing regularly [8], and doing routine partial water changes with dechlorinated water [6]. Sharp decor, fin-nipping tankmates, and rough netting create the very wounds fungus and columnaris exploit, so avoid injury, quarantine new fish and plants, and treat fin rot early — frayed fin edges are a common launch pad for fungus (betta fin rot).

When to See a Vet

Fish medicine is a real veterinary specialty, and a betta with fungus can often be saved — but some cases need an aquatic veterinarian fast. Reach out if:

• The fuzz is on or around the mouth, or is visibly spreading within a day — that pattern points to columnaris, which Merck says can be acutely fatal and hard to control once obvious [3].
• The growth keeps advancing even after you have corrected water quality and temperature.
• Your betta is also lethargic, off its food, breathing hard, or developing ulcers or red sores — signs the infection may be turning systemic.
• You have been told to use formalin, potassium permanganate, or antibiotics and are unsure of the dose, since these are easy to get dangerously wrong in a small tank.

Frequently Asked Questions

Can betta fungus kill my fish?

Yes, especially the columnaris type. The Merck Veterinary Manual notes that columnaris mortality can be acute and difficult to control once the condition is visible. True water-mold fungus (Saprolegnia) is usually slower and often clears once water quality and temperature are corrected, but any infection left unaddressed can be fatal.

Will aquarium salt cure betta fungus?

Aquarium salt is a popular hobbyist supportive measure, not a veterinary-proven cure, and safe concentrations for bettas are poorly documented in veterinary sources — treat salt doses you read online as hobbyist practice and ask a fish vet first. Merck describes formalin and hydrogen peroxide as FDA-approved against fungus on fish eggs, with use on other fish life stages being extra-label and requiring veterinary oversight — not aquarium salt.

How do I tell betta fungus from ich?

Fungus is a fuzzy, cotton-like tuft, while ich is made of discrete white dots. The Merck Veterinary Manual describes ich (white spot disease) as a ciliated parasite that appears as white dots on the skin, in contrast to the grayish-white, cotton-like growths of water-mold fungus. If it looks like scattered grains of salt rather than fuzz, it is ich.

What temperature should my betta's water be?

Bettas are tropical and need a heater. A fish veterinarian's care guide lists 78–82°F (25–28°C), while PetMD and the University of Illinois College of Veterinary Medicine give 76–81°F. A stable temperature in that range supports the immune system, whereas an unheated bowl that drops below it is a classic trigger for water-mold fungus.

Does "mouth fungus" mean my betta really has a fungus?

Usually not. In tropical fish, "cotton wool disease" or "mouth fungus" is most often columnaris — a bacterium, according to both a 2013 Veterinary Research review and the Merck Veterinary Manual — so a cottony mouth patch typically needs an antibacterial, not an antifungal.

References

1. Merck Veterinary Manual. Mycotic Diseases of Fish. https://www.merckvetmanual.com/exotic-and-laboratory-animals/aquarium-fish/mycotic-diseases-of-fish
2. Merck Veterinary Manual. Bacterial Diseases in Aquaculture. https://www.merckvetmanual.com/exotic-and-laboratory-animals/aquaculture/bacterial-diseases-in-aquaculture
3. Merck Veterinary Manual. Bacterial Diseases of Fish. https://www.merckvetmanual.com/exotic-and-laboratory-animals/aquarium-fish/bacterial-diseases-of-fish
4. Merck Veterinary Manual. Parasitic Diseases of Fish. https://www.merckvetmanual.com/exotic-and-laboratory-animals/aquarium-fish/parasitic-diseases-of-fish
5. Declercq AM, et al. Columnaris disease in fish: a review with emphasis on bacterium-host interactions. Veterinary Research. https://pmc.ncbi.nlm.nih.gov/articles/PMC3648355/
6. University of Illinois College of Veterinary Medicine. Your Betta Needs More Than a Bowl. https://vetmed.illinois.edu/pet-health-columns/betta-fish/
7. Fish Vet (cafishvet.com). Standard Care of Betta Fish. https://cafishvet.com/betta-care/
8. PetMD. Betta Fish Care Sheet. https://www.petmd.com/fish/betta-fish-care-sheet
9. Merck Veterinary Manual. Mycotic Diseases in Aquaculture. https://www.merckvetmanual.com/exotic-and-laboratory-animals/aquaculture/mycotic-diseases-in-aquaculture

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