Aquarium Fungus Types and Treatment Guide

Keeping a healthy, thriving aquarium requires diligence and proper care. One of the most common issues aquarium owners face is fungal growth. Fungus in an aquarium may appear as white cotton-like tufts, slimy film on decorations, or woolly patches on a substrate.

While some fungal growth is normal, an overabundance can indicate underlying water quality issues. Certain types of fungus can also infect fish eggs and attack fish tissue, leading to disease. That’s why it’s crucial to identify and treat aquarium fungus promptly.

This comprehensive guide will explore the various types of aquarium fungus, how to diagnose fungal infections in fish, effective treatment options, and prevention through proper aquarium husbandry.

What is Aquarium Fungus?

Fungus refers to a diverse group of organisms in the biological kingdom Fungi. Over 100,000 species exist worldwide, both terrestrial and aquatic.

Fungi are spore-forming, eukaryotic organisms that feed by absorbing nutrients from organic matter. Since aquarium substrates, decorations, and fish waste provide ideal growing conditions, fungal growth is common in home aquariums.

While small amounts of fungus help break down waste, excessive growth indicates an imbalance. The fungus can rapidly multiply and smother aquarium surfaces when water quality declines.

Aquarists can stay vigilant and take prompt action by learning to recognize the most common aquarium fungus types.

Types of Aquarium Fungus

Many varieties of fungus can thrive underwater. The most prevalent types seen in home freshwater and saltwater aquariums include:


One of the most notorious genera, Saprolegnia fungal species, is ubiquitous in freshwater habitats. Saprolegnia appears as white, cotton-like growths on submerged surfaces.

This fungus often colonizes dead organic matter like decaying plants. However, some Saprolegnia species parasitize live fish. Fish eggs are especially vulnerable to Saprolegniosis infections.


Closely related to Saprolegnia, Achlya species also plague freshwater aquariums. This fungus resembles Saprolegnia, forming white or gray tufts on decor.

Achlya spreads rapidly, making it a common culprit in fungal outbreaks. It can smother fish eggs or attack injured tissue.


Another egg-loving fungus, Lagenidium is a scourge for fish breeders. This fungus excels at sensing chemical cues from fish eggs, rushing in to colonize.

Lagenidium coats eggs in a cottony mass, inhibiting oxygen and nutrient absorption. This quickly kills developing embryos.


While less common, Chytridiomycota fungi can infect certain species like amphibians. In aquariums, Chytrid primarily grows on inert surfaces like driftwood.

It often appears as a slimy film that spreads across decorations and substrate. Chytrid is not directly fatal to fish but indicates poor water quality.


A few Aspergillus molds can survive underwater. Aspergillus usually manifests as woolly green to black growths on decor in aquariums.

While not inherently dangerous, Aspergillus can be difficult to eradicate once established fully. Manual removal and antifungals are required.


Oomycetes like water molds are not true fungi but share similar traits. These organisms thrive in fresh and saltwater tanks, coating surfaces in a furry mat.

Many species directly attack live plants, fish, and invertebrates. Oomycetes require potent medications like copper treatments to eliminate.

Signs of Aquarium Fungus

Recognizing the signs of fungal overgrowth is an essential aquarium maintenance skill. Be on the lookout for these common indicators:

  • White cottony tufts adhering to decor, substrate, and tank walls
  • Gray, woolly patches resembling mold on driftwood and other surfaces
  • Slimy biofilm spreading across aquascape features
  • Fuzzy growths encapsulating fish eggs
  • Yellowing, dying plant leaves caused by parasitic fungi
  • Ulcers, lesions, or frayed fins on fish due to fungal infections

Rapid fungal expansion often coincides with water quality issues. For example, high nitrate and phosphate levels provide prime conditions for fungal blooms.

Therefore, the appearance of excessive fungus should prompt aquarists to test parameters like pH, ammonia, and oxygenation thoroughly. Addressing underlying problems is key to fixing fungal overgrowth.

Potential Risks of Aquarium Fungus

For fish and invertebrates, fungi pose multiple risks including:

  • Smothering – Fast-growing fungal mats can overtake tank surfaces, choking out essential oxygen and CO2 exchange. This creates a toxic environment.
  • Tissue Destruction – Some aggressive fungi directly attack living organisms. Fungal infections eat away at fish flesh, destroying fins and creating open wounds.
  • Egg Fouling – Fish eggs are extremely vulnerable to fungi like Lagenidium and Saprolegnia. A fungal attack can destroy an entire spawn within hours.
  • Toxin Release – Certain fungi release dangerous metabolites as they decompose organic matter. These compounds may irritate fish gills and skin.
  • Immune Weakening – Chronic fungal infections tax fish immune systems, leaving them open to potentially deadly opportunistic bacteria and parasites.

While fungi are naturally present in aquariums, excessive growth poses a clear hazard. Taking action at the first signs of overgrowth is crucial.

Treating Aquarium Fungus

If aggressive fungus takes hold in a tank, prompt treatment is required to protect inhabitants and restore balance. Here are some effective remedy options:

Manual Removal

Manually eliminating fungus using an algae scraper, siphon, or by removing infected decor provides immediate relief. This eliminates fungal growth directly.

However, manual removal only treats the visible fungus. To prevent rapid regrowth, chemical or natural treatments are also needed.

Salt Baths

For fish suffering from external fungal lesions, a salt bath can help. Dissolve 1-3 tablespoons of aquarium salt per gallon of water in a quarantine tub.

Dip the infected fish for 5-10 minutes to inhibit fungal growth and allow the fish to heal. Do not use table salt, which contains additives.

Potassium Permanganate

This common aquarium medication oxidizes organics, killing fungus on contact. It is especially effective against Saprolegnia and Achlya.

Use potassium permanganate as a spot treatment or tank-wide dip at 0.1-0.2 ppm. Remove invertebrates first, as they are sensitive.

Methylene Blue

Another popular antifungal, methylene blue treats external fungal infections through direct contact. It is gentle enough to treat eggs and sensitive fish species.

Bathe fish in a methylene blue solution of 1-3 drops per gallon for 30-60 minutes daily until fungus recedes.

Malachite Green

Malachite green works similarly to methylene blue but is more potent. It should not be used to treat scaleless or young fish due to potential toxicity.

This dye treats tough fungal infections on adults at 0.1-0.2 ppm but must be removed after 5-7 days of treatment.

Aquarium Salt

Aquarium salt (sodium chloride) can destroy fungus through osmotic pressure for freshwater tanks. Start with 0.5 teaspoons per gallon.

Salt also helps injured fish retain fluids and heal. Increase salinity gradually over several days to avoid shocking fish.


Prescription antibiotics like chloramphenicol, neomycin, and erythromycin are powerful antifungals. They are often used to treat stubborn Saprolegnia infections.

However, antibiotics affect all bacteria and should only be used as a last resort. They may crash the nitrogen cycle.

Hydrogen Peroxide

Low doses of hydrogen peroxide directly oxidize fungus, helping lift it away from surfaces and fish. For mild cases, use 1-2 ml per 10 gallons.

Turn off filtration while treating. Peroxide breaks into oxygen and water within several hours, posing minimal risks.

Aquarium Plant Removal

Since dying and decaying plant matter fuels fungal blooms, pruning away infected leaves and plants helps limit fungal food sources.

Combine with a blackout period of 3-4 days to allow remaining plants to recover free of fungal attack.

Water Changes & Gravel Vacuuming

Clean, well-oxygenated water inhibits fungal growth. Perform 25-50% water changes at least twice weekly during an outbreak while siphoning waste from substrate.

UV Sterilization

UV sterilizers zap free-floating fungus spores with lethal shortwave radiation as water flows through. They also kill bacteria and algae.

Use a properly sized UV unit like the Green Killing Machine to prevent fungal spores from colonizing surfaces.

Aquarium Hygiene

Preventing fungal growth through proper aquarium hygiene is key. Follow these tips:

  • Quarantine new plants and fish for 2-4 weeks before adding to the main tank
  • Disinfect plants with dilute bleach or potassium permanganate dips
  • Use hospital/quarantine tanks to isolate sick fish for treatment
  • Remove excess food within 1-2 hours to avoid decay
  • Test water parameters weekly and correct issues immediately
  • Avoid overstocking to prevent ammonia and nitrate spikes
  • Clean filters monthly and replace filter media regularly
  • Use air pumps, powerheads, and surface agitation to maintain oxygen saturation
  • Limit lighting to 6-8 hours daily to discourage fungal/algal growth
  • Remove deceased fish, infected eggs, and rotting plant matter promptly
  • Sterilize aquascaping tools, nets, and decor before each use

Natural Aquarium Fungus Remedies

In addition to traditional medications, many natural substances can help combat aquarium fungus. Natural treatments are gentle yet effective alternatives:

Indian Almond Leaves

Leaves from Terminalia catappa trees release tannins and other antifungal compounds as they steep in the tank. These also benefit fish health.

Salt Baths

A natural salt bath with sodium chloride or Epsom salt pulls fluids from fungus through osmosis, dehydrating and killing fungal hyphae.

Garlic Guard

Garlic contains allicin, ajoene, and other antifungal sulfur compounds. Use garlic extract like Seachem Garlic Guard to treat mild fungal infections safely.

Aquarium Salt

Natural sea salt raises salinity levels, creating a hostile environment for freshwater fungus species. Use aquarium-grade salt only.

Activated Carbon

Carbon filtration removes organic particulates, eliminating free-floating fungal spores and potential food sources.

Tea Tree Oil

The antiseptic terpenes in tea tree oil kill fungi and bacteria alike. Use just 2-3 drops per 10 gallons for an effective remedy.

Hydrogen Peroxide

Low peroxide concentrations oxidize fungus directly. Higher doses can harm fish, so only start with 1-2 ml per 10 gallons.


Light deprivation stops photosynthetic fungi species like water molds in their tracks. Black out the tank for 3-4 days to treat blooms.

Aquarium Plant Removal

Since dying leaves fuel fungal growth, pruning away infected leaves improves plant health and limits food sources.

Fungus vs Algae: How to Tell the Difference

Green hair-like growths in an aquarium often prompt panic about a fungal outbreak. However, these growths are usually harmless algae. Here’s how to identify the difference:


  • Fungus – Fluffy cotton-like tufts or slimy patches, often white/gray
  • Algae – Soft green filaments, sometimes resembling wet hair
  • Texture
  • Fungus -looks like mold, felt-like
  • Algae – slimy and easily rubbed off surfaces

Growth Pattern

  • Fungus – Grows in discrete tufts, not evenly distributed
  • Algae – Spreads evenly across surfaces in sheets


  • Fungus – Rapid expansion across surfaces in days
  • Algae – Gradual accumulation over weeks to months

Fish Health Impact

  • Fungus – Can infect fish tissue, attack eggs, or smother tank
  • Algae – Mostly aesthetic nuisance, not directly harmful


  • Fungus – Usually requires medications to eradicate fully
  • Algae – Can be manually scrubbed away or controlled with algaecides

While fungus demands quick action, most types of aquarium algae are easily managed through regular maintenance and tank balance. Don’t panic and nuke healthy tanks unnecessarily!

FAQs About Aquarium Fungus

How do I get rid of fungus in my aquarium?

Start by testing and improving water quality, manually removing visible fungus, and using medications like methylene blue or malachite green. Perform frequent water changes and clean all surfaces to remove spores. Address the root cause like high organics to prevent recurrence.

What does aquarium fungus look like?

Fungus often resembles cotton, fuzzy mold, or grey/white slime. Common forms include Saprolegnia appearing as white tufts on decor and fish or Oomycetes coating surfaces in woolly films. Rapid spreading mold-like growth indicates fungus.

Is fungus in fish tank bad?

In small amounts, some fungus can help break down organics. However, Saprolegnia, Achlya, Lagenidium blooms and other aggressive fungi can harm fish eggs, smother tank surfaces, and infect fish. These types require prompt treatment.

How do I disinfect aquarium decor for fungus?

Wash decorations vigorously in tap water, scrubbing off any residue or growths. Rinse well. Soak decor for 5-10 minutes in a mild bleach solution (1 part bleach to 10 parts water), potassium permanganate dip, or vinegar. Rinse thoroughly before returning items to the tank.

How do I disinfect my aquarium after fungus?

Tear down the tank completely and wash all surfaces with hot water to remove spores. Scrub off any visible biofilm or growths. Soak decorations as described above. Discard filter media. Clean the empty tank with a bleach solution before refilling. Avoid cross-contamination between tanks.


While dealing with aquarium fungus may seem daunting, this guide provides the knowledge to protect your tank inhabitants. You can maintain a thriving fungus-free habitat by learning to quickly recognize fungal growth, utilizing treatment options safely, and preventing blooms through proper care.

With vigilance and prompt action at the first sign of problems, fish and invertebrates can avoid the harmful effects of unchecked fungal infections. Use the treatment and control methods outlined here to get any outbreak under control.