Betta Fish Fungal Infections: Symptoms, Causes & Treatment

Q: How do I know if my betta has a fungal infection or just fin rot?

Fin rot typically presents as fraying, discoloration (often turning the fin edge brown, black, or translucent), and a gradual 'shortening' of the fin from the edges inward — it's primarily a bacterial issue affecting fin tissue itself. A fungal infection, by contrast, usually appears as distinct cottony or fuzzy white/grey growths sitting on top of the skin or fins, almost like tufts of cotton wool stuck to the fish. In practice, the two often occur together — fin rot damages tissue and creates an entry point, and a secondary fungal infection takes hold at the damaged site. If you see both fraying fin edges and cottony growths, you may be dealing with both issues at once, and treatment should address each.

Q: Can fungal infections in bettas spread to other fish in the tank?

The fungal organisms themselves (commonly Saprolegnia and related water molds) are often present at low levels in most aquarium water and primarily affect fish that are already compromised — through injury, stress, or poor water quality — rather than spreading aggressively between healthy fish the way some bacterial or parasitic diseases do. That said, if a betta is showing a fungal infection, it's worth treating the underlying water quality issue (if one exists) for the whole tank, since whatever stressed the betta enough to allow the infection to take hold may affect other fish too, even if they don't develop visible fungus themselves. If keeping the betta with tank mates, monitor everyone closely during treatment.

Q: How can I prevent fungal infections from recurring?

Since fungal infections in bettas are almost always secondary — opportunistic infections taking advantage of an injury, stressed immune system, or poor water quality — prevention is mostly about addressing those underlying factors rather than the fungus itself. Maintain stable water parameters (0 ppm ammonia/nitrite, appropriate temperature for bettas at 76-82°F, as covered in our betta care guide); use smooth decor and gentle filtration to minimize fin damage, especially for long-finned varieties; and quarantine any new fish or plants before adding them to an established tank, since quarantine is also the standard recommendation for managing risk with other incurable or hard-to-treat conditions discussed on this site, like dwarf gourami iridovirus.

By Hektor Jorgo Published December 27, 2022 Updated June 13, 2026 5 min read

A betta fish with a small patch of cottony fungal growth near its fin edge

Quick Facts

Main Symptom: Cottony, white or grey tufts on the body, fins, or mouth
Primary Cause: Secondary infection following injury, fin damage, or poor water quality
Often Confused With: Columnaris (bacterial) and other 'cotton wool'-appearing conditions — see our betta skin peeling guide
Standard Treatment: Antifungal medication plus water quality correction; aquarium salt for mild cases
Common Entry Point: Untreated fin rot or physical injury from decor
Prognosis: Good with early treatment; more guarded if infection reaches the body or mouth
Prevention: Stable water quality, smooth decor, quarantine for new fish

A cottony, white or grey tuft growing on a betta's fin or body is one of the more alarming things a keeper can spot — and also one of the more treatable, provided it's caught early and the underlying cause is addressed alongside the visible growth. Here's what fungal infections in bettas actually look like, why they happen, and how to treat them.

Short Answer

Fungal infections in bettas typically appear as cottony or fuzzy white-to-grey growths on the body, fins, or sometimes the mouth, and are almost always secondary — meaning they take hold because the fish is already compromised in some way, usually through a physical injury (including fin rot damage), or a period of poor water quality. Treatment involves an antifungal medication or, for mild localized cases, an aquarium salt bath, combined with correcting whatever water quality or injury issue let the infection start in the first place. Treating only the visible growth without addressing the underlying cause is a common reason these infections recur.

What Fungal Infections Look Like on Bettas

The hallmark sign is a cottony, fuzzy, or thread-like growth, usually white or grey, sitting on the surface of the skin or fins — often described as looking like a tuft of cotton wool stuck to the fish. Common locations include:

Fin edges, especially where fin rot has already caused damage
The body, particularly around any existing wound or scrape
The mouth, sometimes following "mouth fungus" — though a condition with this name is frequently caused by bacteria (columnaris) rather than true fungus, which is part of why these conditions are so often confused (see below)

Affected bettas may also show secondary signs of stress — reduced appetite, clamped fins, or lethargy — though these are non-specific and occur with many different health issues.

Common Causes

True fungal infections in aquarium fish are typically caused by water mold organisms (commonly in the genus Saprolegnia), which are present at low levels in many aquarium environments without causing problems in healthy fish. They become an issue when a fish is compromised by:

Physical injury — torn fins from sharp decor, aggressive tank mates, or rough handling during netting/transport create an entry point
Untreated fin rot — fin rot damages tissue first, and a secondary fungal infection often follows at the damaged site
Poor water quality — elevated ammonia, nitrite, or simply unstable parameters stress a betta's immune system and slime coat, both of which normally help keep opportunistic organisms in check
Temperature outside the recommended range — bettas kept too cold (common in unheated setups, as discussed in our betta care guide) are more broadly susceptible to a range of health issues, including fungal infections

Fungal Infections vs. Other Look-Alike Conditions

"Fungus" is one of the most overused terms in fishkeeping, often applied to several genuinely different conditions that happen to share a fuzzy or cottony appearance:

Columnaris ("cotton wool disease" or "mouth fungus") is actually a bacterial infection, not fungal, but produces a similar cottony appearance — often around the mouth, gills, or as patches on the body. Because the appearance overlaps so much with true fungal infections, but the treatment (antibacterial rather than antifungal medication) is different, misidentification is common. Skin lesions associated with columnaris are covered in more detail in our betta skin peeling guide.
"Neon tetra disease" in tetras is a completely different example of a condition that gets lumped in with "fungus" due to surface appearance, despite being caused by a microsporidian parasite with no relation to true fungal infections — covered in our neon tetra fungus guide, which goes into more depth on why "fungus-like appearance" and "actual fungus" are often two different things.

If a treatment for true fungal infections isn't producing improvement after a reasonable period, it's worth considering whether the underlying issue might actually be bacterial (columnaris) rather than fungal, since the two require different medications.

Treatment Options

For a localized, early-stage fungal patch:

Aquarium salt baths — a brief, supervised dip in an aquarium-salt solution (not added to the main tank long-term) can help with mild, localized infections, though severity and the betta's overall condition should guide whether this is appropriate
Antifungal medications — commercially available products formulated for aquarium fish, dosed per label instructions, are the standard treatment for more established infections

For any fungal infection, regardless of severity:

Test water parameters — ammonia, nitrite, nitrate, pH, and temperature. Correct anything outside the target ranges in our betta care guide (76-82°F, 0 ppm ammonia/nitrite)
Check for the injury source — sharp decor, aggressive tank mates, or rough fin damage from strong filter flow (a particular risk for long-finned varieties)
Consider whether fin rot preceded the fungus — if so, both conditions may need addressing

Prevention

Because fungal infections are overwhelmingly secondary, prevention is mostly about the same fundamentals that prevent a wide range of betta health issues:

Maintain stable water quality (0 ppm ammonia/nitrite, appropriate temperature)
Use smooth decor and gentle filtration, especially for long-finned varieties prone to fin damage
Address fin rot promptly if it appears, rather than letting it progress
Quarantine new fish and plants before introducing them to an established tank

Quick Reference

Cottony, white/grey tufts on body, fins, or mouth are the hallmark sign of a fungal infection
Fungal infections are almost always secondary to injury, fin rot, or poor water quality
Treatment = antifungal medication (or salt bath for mild cases) + fixing the underlying cause
Columnaris (bacterial) can look similar but needs different treatment — don't assume "cotton wool" always means fungus
Test and correct water parameters as part of any treatment plan
Prevention centers on stable water quality, smooth decor, and quarantine for new fish
If antifungal treatment isn't helping, consider a bacterial cause (columnaris) instead

Frequently Asked Questions

How do I know if my betta has a fungal infection or just fin rot?

Fin rot typically presents as fraying, discoloration (often turning the fin edge brown, black, or translucent), and a gradual 'shortening' of the fin from the edges inward — it's primarily a bacterial issue affecting fin tissue itself. A fungal infection, by contrast, usually appears as distinct cottony or fuzzy white/grey growths sitting on top of the skin or fins, almost like tufts of cotton wool stuck to the fish. In practice, the two often occur together — fin rot damages tissue and creates an entry point, and a secondary fungal infection takes hold at the damaged site. If you see both fraying fin edges and cottony growths, you may be dealing with both issues at once, and treatment should address each.

Can fungal infections in bettas spread to other fish in the tank?

The fungal organisms themselves (commonly Saprolegnia and related water molds) are often present at low levels in most aquarium water and primarily affect fish that are already compromised — through injury, stress, or poor water quality — rather than spreading aggressively between healthy fish the way some bacterial or parasitic diseases do. That said, if a betta is showing a fungal infection, it's worth treating the underlying water quality issue (if one exists) for the whole tank, since whatever stressed the betta enough to allow the infection to take hold may affect other fish too, even if they don't develop visible fungus themselves. If keeping the betta with tank mates, monitor everyone closely during treatment.

What's the best way to treat a fungal infection on a betta?

Treatment has two parts, and skipping the second one is the most common reason fungal infections come back. First, treat the visible infection — commercially available antifungal medications formulated for aquarium use are the standard approach, dosed according to the product's instructions; aquarium salt baths (brief, supervised dips in a salt solution, not adding salt to the main tank long-term) are sometimes used for milder, localized cases. Second, and just as important, identify and fix whatever allowed the infection to take hold in the first place — test water parameters (ammonia, nitrite, nitrate, pH, temperature) and correct any issues, check for sharp decor that may have caused the original injury, and consider whether the betta's fins were already compromised by fin rot before the fungal infection appeared.

How can I prevent fungal infections from recurring?

Since fungal infections in bettas are almost always secondary — opportunistic infections taking advantage of an injury, stressed immune system, or poor water quality — prevention is mostly about addressing those underlying factors rather than the fungus itself. Maintain stable water parameters (0 ppm ammonia/nitrite, appropriate temperature for bettas at 76-82°F, as covered in our betta care guide); use smooth decor and gentle filtration to minimize fin damage, especially for long-finned varieties; and quarantine any new fish or plants before adding them to an established tank, since quarantine is also the standard recommendation for managing risk with other incurable or hard-to-treat conditions discussed on this site, like dwarf gourami iridovirus.

Sources & Further Reading

About the Author: Hektor Jorgo

Co-Founder & Marine Biologist

Hektor is a co-founder of Sea Life Planet and has kept reef and freshwater aquariums for over 15 years. He holds a background in marine biology and focuses on species care accuracy, water chemistry, and tank husbandry.