Saprolegnia and Saprolegniasis in aquaculture and ornamental Fish: A comprehensive guide to advanced causes, symptoms, treatment, and prevention

Saprolegnia, commonly known as “water mold” or the causative agent of “cotton wool disease,” is a microorganism that represents a significant threat to both the aquaculture industry and ornamental fish enthusiasts. Although often called a fungus, it is important to note that Saprolegnia belongs to the Oomycetes group, filamentous organisms distinct from true fungi. This opportunistic pathogen can affect a wide range of aquatic organisms, including fish, amphibians (such as axolotls), and, to a lesser extent, crustaceans, causing the disease known as Saprolegniasis.

In freshwater fish, mainly ornamental fish, and in estuarine fish (Vajargah and Majidiyan, 2022), fungi of the genus Saprolegnia are the cause of the disease known as Saprolegniasis or “cotton wool disease.” The presence of Saprolegniasis can lead to considerable economic losses in fish farms and is a major concern for aquarium keepers. Among the mycotic diseases of freshwater fish, Saprolegniasis is one of the diseases of greatest economic importance (Lone and Manohar, 2018). Understanding its causes, identifying its symptoms in a timely manner, and knowing the treatment and prevention strategies are crucial for maintaining the health of aquatic populations.

What is Saprolegnia and how does it cause Saprolegniasis?

The genus Saprolegnia includes filamentous oomycetes that are naturally found in most freshwater ecosystems. These microorganisms act mainly as saprotrophs, decomposing dead organic matter, but under certain conditions, they become necrotrophs, capable of infecting the living tissues of weakened or previously injured aquatic animals. The characteristics of Saprolegnia sp. include the formation of a cotton-like mass of mycelium (a set of hyphae) on the host.

Taxonomy of Saprolegnia

Kingdom: Chromista

Phylum: Oomycota

Order: Saprolegniales

Family: Saprolegniaceae

Genus: Saprolegnia

Species: Saprolegnia aenigmatica, S. anisospora, S. anomalies, S. asterophora, S. australis, S. bulbosa, S. delica, S. diclina, S. eccentrica, S. ferax, S. cf. ferax, S. furcata, S. hypogyna, S. lapponica, S. litoralis, S. longicaulis, S. megasperma, S. milanezii, S. mixta, S. monilifera, S. monoica, S. multispora, S. oliviae, S. parasitica, S. polymorpha, S. racemosa, S. salmonis, S. semihypogyna, S. subterranea, S. terrestris, S. torulosa, S. truncata, S. turfosa, S. unispora

Common names in Spanish: moho de agua or moho de algodón

Common names in English: water mold or cotton mold.

It is important to note that Saprolegnia is a genus that includes many species of fungi living in fresh or brackish water, and they can infect fish, amphibians, and crustaceans (Kumar et al, 2020). Among the most relevant species are:

• Saprolegnia parasitica: Considered the most virulent species and commonly isolated in freshwater fish infections, both in aquaculture (especially salmonids) and in ornamental fish. According to Kumar et al., (2020) S. parasitica is one of the most severe fish pathogens, responsible for large losses in the aquaculture industry worldwide.
• Saprolegnia diclina: Frequently associated with infections in fish eggs and also pathogenic to amphibians. Hendrik et al., (2013) reported that S. diclina is an oomycete that infects the eggs of aquatic animals.
• Other species such as Saprolegnia ferax and Saprolegnia australis have also been identified in disease outbreaks.

Infection mechanism of Saprolegnia

The infection mechanism of Saprolegniasis begins when the mobile zoospores of Saprolegnia (its infective form) find a susceptible host, generally through a skin wound, damaged gills, or in fish with a compromised immune system. Various studies have reported that some species, like S. parasitica, use specific proteins to adhere to the host’s tissues. Once attached, the zoospores encyst and germinate, developing hyphae that penetrate the tissues, releasing enzymes that cause damage and cellular necrosis, which manifests as the typical cotton-like lesions.

Trusch et al. (2018) provide a detailed model of how an animal-pathogenic oomycete secretes an effector protein (SpHtp3) that self-translocates into host cells via a gp96-like receptor and is released into the cytoplasm by another effector protein (SpHtp1) to exert its nuclease function.

Life cycle of Saprolegnia: An adaptable pathogen

Understanding the life cycle of Saprolegnia is fundamental to understanding its persistence and infection capacity. This oomycete has two main reproductive phases:

• Asexual reproduction: This is the most common form of propagation and dissemination.
  • The somatic hyphae develop elongated structures called zoosporangia.
  • Within the zoosporangia, numerous primary zoospores are formed, which are biflagellated (with two flagella that allow them mobility in the water).
  • These zoospores are released and swim actively in search of a suitable substrate (organic matter or a host). If they do not find one, they can encyst and then release a secondary zoospore (a phenomenon known as polyplanctism), increasing their chances of survival and dispersal. This phase is crucial for the rapid colonization and development of Saprolegnia’s reproduction.
  • Keywords: saprolegnia zoospores, saprolegnia flagella.
• Sexual reproduction: Occurs in less favorable environmental conditions or when different compatible strains come into .
  • Male (antheridia) and female (oogonia) sexual structures are formed.
  • Fertilization leads to the formation of oospores, which are thick-walled, highly resistant spores.
  • The oospores can remain dormant for long periods, surviving adverse conditions (low temperatures, partial desiccation) and germinating when conditions become favorable again. This is one of the reasons why Saprolegnia is so persistent in aquatic environments.

The structure of Saprolegnia, with its non-septate (coenocytic) hyphae and massive production of zoospores, contributes to its success as an opportunistic pathogen. A detailed visual description or a diagram of Saprolegnia would show these branched hyphae and the different types of spores.

Predisposing factors and common causes of Saprolegniasis

Saprolegniasis rarely affects healthy fish in optimal environments. Its appearance is strongly linked to factors that weaken the host or deteriorate its environment; based on the research of Tedesco et al. (2022), Sanders (2022), the following causes are reported:

• Poor water quality: This is the main trigger. Pavić et al. (2022) reported on how the physicochemical parameters of the water, such as calcium and electrical conductivity, and fluoride, as well as the host’s health status (the presence of lesions increases the load), influence the abundance of this opportunistic pathogen in freshwater ecosystems and aquaculture.
  • Inadequate temperatures, especially sudden drops or temperatures that are too cold for tropical species.
  • High levels of ammonia, nitrites, or low dissolved oxygen concentration.
  • Inadequate or fluctuating pH.
  • High load of decomposing organic matter, which serves as an initial nutrient for Saprolegnia and deteriorates the overall water quality.
• Stress in fish: Stress suppresses the immune system, making them vulnerable.
  • Rough handling (capture, transport).
  • High population density (overcrowding).
  • Incorrect acclimation to a new aquarium.
  • Aggression from other fish.
  • Sudden environmental changes.
• Previous injuries: Any breach in the skin or mucus protective barrier.
  • Physical wounds, scrapes against rough objects.
  • Infections by ectoparasites (like Ichthyophthirius or trematodes) that cause wounds.
  • Primary bacterial infections that ulcerate the skin.
• Compromised immune status:
  • Poor nutrition or diets deficient in essential vitamins.
  • Concomitant diseases.
  • Age (very young or old fish may be more susceptible).
• Natural presence of the pathogen: The spores of Saprolegnia are ubiquitous in most freshwater bodies. The disease manifests when the aforementioned factors allow the pathogen to overcome the fish’s defenses. The question of where Saprolegnia is found has a simple answer: in virtually all freshwater aquatic environments.

Symptoms and clinical signs of Saprolegniasis: Recognizing cotton wool disease

Early identification of Saprolegniasis symptoms is vital for successful treatment. The most characteristic signs of “cotton wool disease” include:

• Cotton-like appearance: This is the most evident sign. Tufts or patches with a cotton-like appearance are observed, white, grayish, or even brown (if aquarium debris adheres to them), on the skin, fins, gills, or eyes of the fish. These are the hyphae of the oomycete growing on the affected tissue. Keywords: saprolegnia in fish, saprolegnia fungus, trout water mold.
• Cutaneous lesions:
  • Initially, they may appear as pale or discolored areas on the skin.
  • They progress to erosions, ulcers, and, in advanced cases, tissue necrosis.
  • There may be localized hemorrhages around the lesions.
  • The fins may appear frayed or eroded.
• Egg infestation: Infected eggs become covered with a cottony mycelial mass, turning opaque and leading to the death of the embryo. This is a serious problem in fish farming and for ornamental fish breeders.
• Behavioral symptoms:
  • Lethargy or apathy, the fish appears less active than normal.
  • Loss of appetite.
  • Slow movements or erratic swimming.
  • Isolation from the group.
  • Some fish may rub against aquarium objects in an attempt to get rid of the parasite in the initial stages.
• Severe cases: Although Saprolegnia is usually a superficial infection, in very weakened fish or with particularly virulent species of Saprolegnia, the hyphae can penetrate deeper into the tissues, reaching the musculature. Systemic infections are rare but, if they occur, are usually rapidly fatal.

Diagnosis of Saprolegniasis: Confirming the infection

While the cotton-like clinical signs are highly suggestive of Saprolegniasis, a precise diagnosis may require additional steps, especially in aquaculture settings or to rule out other conditions:

• Macroscopic examination: Direct observation of the characteristic cottony lesions on the skin, fins, or eggs of the fish is the first step and often sufficient for a presumptive diagnosis in the context of a home aquarium.
• Microscopic examination: This is the most common and accessible confirmation method.
  • A sample (scraping or smear) is taken from the cottony lesion.
  • It is placed on a slide with a drop of water or saline solution and observed under the microscope.
  • The characteristic hyphae of Saprolegnia are sought: large, wide, branched, and notably non-septate (coenocytic). Zoosporangia releasing zoospores can also be observed at the end of some hyphae.
• Culture and isolation: In laboratories or for research, samples can be cultured on specific agar media (such as Sabouraud dextrose agar or cornmeal agar) to isolate the oomycete and allow for a more precise species identification.
• Molecular techniques: For a definitive species-level identification and in epidemiological studies, techniques such as the Polymerase Chain Reaction (PCR) are used to amplify and sequence specific regions of the oomycete’s DNA. Environmental DNA (eDNA) is also being explored to detect the presence of Saprolegnia in the water before visible outbreaks occur. Korkea et al (2022) validated a previously published real-time quantitative PCR (qPCR) assay to confirm the presence of S. parasitica in fish and water by quantifying environmental DNA (eDNA); while Pavić et al. (2022) developed a rapid, sensitive, and specific droplet digital PCR (ddPCR) assay to detect and quantify the pathogen Saprolegnia parasitica in environmental samples.

Differentiating Saprolegniasis from columnar bacterial infections (which sometimes produce whitish lesions) or from true aquatic fungi (less common as primary pathogens in fish) is important for directing the appropriate treatment.

Aquatic species affected by Saprolegnia: A wide range of hosts

Oomycetes of the genus Saprolegnia cause the disease saprolegniasis in fish, with considerable losses, especially in freshwater (Korkea et al., 2022) or brackish water (<2.8 ppm salinity) aquaculture, because damage to the skin or gills exposes the fish to secondary attacks by bacteria and other fungi (Sanders, 2022).

Saprolegnia is not selective with a single type of fish; it can affect a great variety of aquatic species, especially under conditions of stress or injury.

• Freshwater fish:
  • Salmonids: They are particularly susceptible, including trout and salmon. Saprolegniasis is one of the main causes of losses in freshwater salmon farming.
  • Cyprinids: Carp (including Koi) and goldfish are frequently affected, especially in ponds or aquariums with poor water quality.
  • Tilapias: Saprolegnia in tilapia is a recognized problem in aquaculture farms.
  • Tropical ornamental fish: A wide range, including betta, guppy, tetras, cichlids, among others, can develop the disease if the aquarium conditions are not optimal.
  • Catfish and other Siluriformes. Rathod et al., (2024) identified Saprolegnia parasitica as a significant pathogen of the basa fish Pangasianodon hypophthalmus, particularly in conditions of low ambient temperature that favor fungal growth and precipitate winter outbreaks.
• Amphibians: Many amphibians are known to suffer the death of their embryos as a consequence of saprolegnia infections (Fernández et al., 2008); the fungus colonizes pre-existing skin lesions in aquatic amphibians (O’Rourke and Rosenbaum, 2015).
  • Axolotls (Ambystoma mexicanum): They are extremely sensitive to saprolegnia in axolotls, and infections can be rapidly fatal if not treated. The lesions caused by saprolegnia are usually raised, dark nodules; however, they can be ulcerated and can also cause weakness and weight loss.
  • Frogs and salamanders: Especially their larvae and eggs can be vulnerable. Fernández et al. (2008) report the species S. diclina as the cause of Saprolegnia infections in the embryos of the “natterjack toad” (Bufo calamita) in the mountainous areas of central Spain.
• Fish eggs and embryos: Saprolegnia (especially S. diclina) is a major cause of mortality in the eggs of many fish species, both in nature and in artificial incubators, forming a dense cottony layer that prevents oxygen exchange.
• Crustaceans: Although less common, cases of Saprolegniasis have been reported in freshwater shrimp, usually as a secondary infection to severe stress or injury.

Susceptibility can vary between species and even between individuals of the same species, depending on their general health status and environmental conditions.

Treatment and control of Saprolegniasis: Updated strategies

The treatment of Saprolegnia in fish must be rapid and multifaceted, addressing both the pathogen and the predisposing factors. The treatment for cotton wool disease depends on the type of fish, the severity of the infection, and the size of the infected area. For mild cases, raising the tank temperature and maintaining good water quality is often sufficient. In more severe cases, antifungal medications may be necessary.
Here are various strategies, from immediate measures to specific treatments:

Immediate and Fundamental Measures

• Isolation: Immediately move the affected fish to a hospital tank with clean water and optimal parameters. This prevents the spread of zoospores in the main tank and allows for a more focused treatment.
• Water Quality Improvement: In both the main and hospital tanks, it is crucial to perform partial water changes (25-50%), siphon the bottom to remove detritus and organic matter, and check/correct key parameters (temperature, pH, ammonia, nitrites). Good oxygenation is essential.

Chemical Treatments (Use with caution and according to local legislation)

Salt (Non-iodized sodium chloride)

This is one of the safest and most accessible treatments, especially for ornamental fish. Vajargah and Majidiyan (2022) emphasize that for fish consumed by people (trout, salmon, tilapia, carp, etc.), it is recommended to use only salt. You can perform short baths (10-30 g/L for 15-30 minutes) or maintain a low concentration in the hospital tank (1-3 g/L) for several days. It helps restore the fish’s osmotic balance and has an antifungal effect.

Formalin (Formaldehyde)

Historically widely used in aquaculture, especially for baths (e.g., 150-250 ppm for up to 1 hour). It is effective, but it is a dangerous chemical that must be handled with extreme caution (gloves, goggles, good ventilation). Its use is increasingly restricted in several places due to environmental and health concerns.

Potassium Permanganate (KMnO₄)

It is used in short baths (e.g., 2-4 ppm for 30-60 minutes). It is a strong oxidant that can be effective, but an overdose is toxic. The water will turn pink/purple; if it turns brown quickly, it indicates high organic matter and may require re-dosing or a water change.

Copper Sulfate

Effective against oomycetes, but its toxicity to fish varies enormously with water hardness (alkalinity). It is safer in hard water. Recent research seeks to enhance its effect at low doses by combining it with ionophores. It requires a copper test kit to monitor the concentration. Many invertebrates are extremely sensitive to copper.

Boric Acid

In the case of salmon eggs, Ali et al. (2014) used boric acid as a prophylactic measure and to cure the cotton wool infection in fertilized eggs and larvae with yolk sacs. No signs of saprolegniasis are observed in Nile tilapia (Oreochromis niloticus) treated with boric acid at concentrations above 0.4 g/L (Ali et al., 2019).

Malachite Green

IT IS BANNED in many countries for use in food-producing animals, and its use in ornamental fish is highly discouraged due to its demonstrated carcinogenicity and environmental persistence (Kumar et al., 2020). We mention it here for its historical importance as a very effective treatment, the prohibition of which has driven the search for alternatives.

Commercial antifungal products

There are various commercial formulations designed to treat fungal and oomycete infections in fish. It is crucial to read and follow the manufacturer’s instructions to the letter, including dosage and duration of treatment. Some may contain combinations of the chemicals mentioned or other compounds.

In this regard, Rathod et al. (2024) reported that clotrimazole stands out as a potential veterinary drug to control superficial mycosis due to its significant in vitro inhibitory activity; this product showed significant inhibitory activity against the growth of Saprolegnia parasitica spores and hyphae at a very low concentration of 2 mg L⁻¹.

Other chemical products

Kumar et al. (2020) found that triclosan (approved by the US FDA) was more effective with a minimum inhibitory concentration (MIC100) of 4 μg/ml. Likewise, Tedesco et al. (2018) report that in in vitro tests, benzoic acid and iodoacetic acid showed the best results, and that products based on acetic acid and peracetic acid, in combination with hydrogen peroxide, are promising candidates.

Physical treatments

Heikkinena et al. (2013) reported that high-dose UV irradiation (400 mWs/cm²) of the inflow water significantly decreased the mortality of rainbow trout (Oncorhynchus mykiss) eggs from 77.3% to 14.3% caused by Saprolegnia spp. water mold during a 28-day trial.

Meanwhile, Johari et al. (2016) evaluated the indirect use of silver nanoparticles (AgNPs) in water filters and report that it was significantly effective in preventing fungal infections in semi-recirculating systems for rainbow trout during the incubation period.

Biological treatments

Probiotics

The addition of beneficial bacteria to the water or feed is being investigated as a way to control pathogens and improve the overall health of fish, and could be a preventive strategy against Saprolegniasis. In this regard, Firouzbakhsh et al. (2014) reported that dietary supplementation (1.0 g kg⁻¹) with a synbiotic in rainbow trout (Oncorhynchus mykiss) fry increases growth performance and survival rate against Saprolegnia parasitica infection.

On the other hand, Lone and Manohar (2018) report that Actinobacteria of the genus Frondihabitans (Microbacteriaceae) inhibit the adherence of Saprolegnia to salmon eggs.

Plant extracts and essential oils

Research is exploring natural compounds. For example, linalool, a component of many essential oils, has shown anti-Saprolegnia parasitica activity (Tao et al., 2025); while Tang et al., (2024) reported that linalool demonstrated effective anti-oomycete activity both in vitro and in vivo against Saprolegnia ferax.

Ashraf et al. (2020) report that ethanolic extracts of “pomegranate” Punica granatum and “thyme” Thymus vulgaris exhibited potential efficacy in suppressing the mycelial growth of S. diclina at a concentration of 0.5 mg/ml. In the same vein, the research results of ALsafah and AL-Faragi (2017) indicate that supplementing the feed of common carp (Cyprinus carpio) with “thyme” improves the fish’s immunity when challenged with Saprolegnia.

Meanwhile, Nardoni et al., (2019) report that the essential oils of “Litsea” Litsea cubeba, “lemongrass” Cymbopogon flexuosum, and “bergamot orange” Citrus bergamia, in in vitro tests, may be of interest for controlling Saprolegnia.

Mehrabi et al. (2019) conclude that using 15 grams of Aloe vera (Aloe barbadensis) powder per kilogram of feed in rainbow trout (Oncorhynchus mykiss) reduces the mortality of fish infected with Saprolegnia.

The results of the study by Elgendy et al. (2022) indicate that feeding tilapia with diets supplemented with medicinal plant products, such as Allium cepa (onion), can improve various aspects of fish health and performance.

The study by Aly et al. (2025) concludes that garlic (1.5%) and cinnamon (8.5%) can serve as effective immunostimulants to control fungal infections in fish, specifically in Nile tilapia (Oreochromis niloticus).

Strengthening the immune system

A high-quality diet, rich in vitamins (especially C and E) and other immunostimulants, can help fish resist infections. The main finding of the study by Alafari et al. (2025) was that a dietary combination of selenium nanoparticles (SeNPs) and Vitamin C (SeNPs + VC100 group) synergistically improved the antioxidant capacity, immune response, organ health, growth, and disease resistance in tilapia fish (Oreochromis niloticus).

Management of secondary infections

The lesions caused by Saprolegnia can be colonized by opportunistic bacteria. If signs of bacterial infection are observed (severe redness, septicemia), treatment with antibiotics may be necessary, ideally prescribed by a veterinarian specializing in fish.

It is important to that the cure for Saprolegnia not only depends on the therapeutic agent but also on correcting environmental factors and the overall health status of the fish. The treatment of Saprolegnia in fish must be comprehensive.

Prevention: The best strategy against Saprolegnia

Given that Saprolegniasis is a predominantly opportunistic disease, prevention is, by far, the most effective and economical strategy. The following measures are fundamental:

• Impeccable water quality maintenance:
  • Perform regular partial water changes (weekly or bi-weekly, depending on the biological load).
  • Periodically monitor key parameters: pH, ammonia, nitrites, nitrates, and temperature.
  • Ensure adequate biological, mechanical, and chemical filtration.
  • Siphon the substrate to remove leftover food and feces.
  • Maintain good oxygenation.
• Avoiding stress:
  • Carefully acclimate all new inhabitants to the aquarium.
  • Do not overpopulate the tank. Each fish needs adequate space.
  • Provide an enriched environment with hiding places, if necessary, for the species kept.
  • Handle fish gently (capture, transfers) and minimize time out of the water.
  • Maintain compatibility between species to avoid aggression.
• Optimal nutrition:
  • Offer a varied, balanced, and high-quality diet, specific to the needs of each species.
  • Avoid overfeeding, as uneaten food decomposes and affects water quality.
• Strict quarantine:
  • All new fish, plants, and other organisms should go through a quarantine period (ideally 4 weeks) in a separate tank before being introduced to the main aquarium. This allows for the observation and treatment of possible diseases without risking the existing population.
• Hygiene and disinfection:
  • Regularly clean aquarium equipment (filters, heaters, decorations).
  • Use separate nets and tools for the quarantine and main tanks, or disinfect them carefully between uses.
• Wound prevention:
  • Ensure that aquarium decorations do not have sharp edges.
  • Prevent aggression by keeping compatible species.
• Parasite control: Promptly treat any external parasite infestation, as the wounds they cause are entry points for Saprolegnia.
• Daily observation: Dedicate time each day to observe the behavior and appearance of the fish. Early detection of any abnormality is key to acting quickly.

Impact of Saprolegniasis in aquaculture

Saprolegniasis is not just a problem for home aquarists; it has a considerable economic impact on the worldwide aquaculture industry.

• Direct economic losses:
  • Mortality of fish at all stages of cultivation, from eggs and fry to market-ready adults. Infections in eggs can lead to massive losses in fish farms.
  • Reduction in growth rates and feed conversion in subclinically affected fish.
  • Costs associated with treatment (chemicals, labor).
• Affected aquaculture species: Salmonids (rainbow trout, Atlantic salmon) are especially vulnerable. It also affects the production of tilapia, catfish, carp, and other commercially important species.
• Re-emerging problem: The incidence of Saprolegniasis in aquaculture has increased in some regions following the ban or restriction of highly effective but environmentally problematic treatments, such as malachite green.
• Challenges for control:
  • The ubiquity of Saprolegnia in freshwater environments makes its eradication practically impossible.
  • The need for treatments that are effective, economically viable, and safe for the fish, consumers, and the environment.
  • The development of resistance to some treatments is a constant concern.
• Importance of biosecurity and Good Management Practices: Prevention, through the optimization of water quality, stress reduction, egg disinfection, and careful handling, remains the most important strategy to minimize the impact of Saprolegnia in aquaculture. Research continues to focus on developing more sustainable and effective control methods, including vaccines (although difficult for oomycetes), immunostimulants, probiotics, and the use of natural compounds.

Conclusion

Saprolegnia, that dreaded “water mold,” is a clear example of an opportunistic pathogen. Its presence in the water is almost a constant, but Saprolegniasis only manifests when the fish’s defenses are compromised or their environment deteriorates. The key to combating this disease lies not solely in treatments, although they are necessary in an outbreak, but in proactive and preventive management.

Maintaining exceptional water quality, minimizing fish stress, providing adequate nutrition, and practicing good hygiene in the aquarium or aquaculture facility are the fundamental pillars to prevent “cotton wool disease” from becoming a problem. Attentive observation and quick action at the first signs are crucial to protect the health and well-being of our aquatic organisms. An integrated approach, combining rigorous prevention with informed treatment strategies, is the best defense against Saprolegniasis.

Frequently Asked Questions (FAQ)

• Can Saprolegnia affect humans?
  • No, Saprolegnia is a pathogen of cold-blooded aquatic organisms (fish, amphibians) and is not considered a threat to human health.
• How can I differentiate Saprolegnia from other white spots on my fish?
  • Saprolegniasis typically presents a three-dimensional, cottony appearance, like tufts. Other diseases like Ich (Ichthyophthirius multifiliis) manifest as small, defined, flat white spots, like grains of salt. Columnar bacterial infections may look like whitish patches, but they often have a more eroded or necrotic appearance without such pronounced filamentous growth. A microscopic examination can be definitive.
• Is it safe to use common salt to treat Saprolegnia in all fish?
  • Salt (non-iodized sodium chloride) is an effective and safe first-line treatment for many freshwater fish. However, some species are more sensitive to salt than others (e.g., certain scaleless catfish or some aquatic plants). It is always best to research the salt tolerance of the specific species being kept and to increase the concentration gradually. Short baths in higher concentrations are often safer than maintaining a high concentration in the tank long-term for sensitive species.
• How fast can Saprolegnia act?
  • In conditions unfavorable for the fish and favorable for the oomycete, Saprolegniasis can develop and spread very quickly, sometimes causing extensive damage or death in a matter of 24 to 72 hours, especially in small, weakened fish, or on eggs.
• Can I use human antifungal medications on my fish?
  • It is generally not recommended. The formulations and dosages for humans are very different and could be toxic to fish. It is better to use products specifically designed for aquarium use and to follow the manufacturer’s instructions or consult a veterinarian specializing in fish.

Bibliographic References

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Ali SE, Thoen E, Evensen Ø, Skaar I (2014) Boric Acid Inhibits Germination and Colonization of Saprolegnia Spores In Vitro and In Vivo. PLoS ONE 9(4): e91878. https://doi.org/10.1371/journal.pone.0091878

Ali, S.E., Gamil, A.A.A., Skaar, I. et al. Efficacy and safety of boric acid as a preventive treatment against Saprolegnia infection in Nile tilapia (Oreochromis niloticus). Sci Rep 9, 18013 (2019). https://doi.org/10.1038/s41598-019-54534-y

Aly, S. M., Elatta, M. A., Nasr, A. A., & Fathi, M. (2025). Efficacy of garlic and cinnamon as an alternative to chemotherapeutic agents in controlling Saprolegnia infection in Nile tilapia. Aquaculture and Fisheries, 10(1), 105-114. https://doi.org/10.1016/j.aaf.2023.07.010

ALsafah Ameer and Jamal AL-Faragi. 2017. Influence of thyme (Thymus vulgaris) as feed additives on growth performance and antifungal activity on Saprolegnia spp. in Cyprinus carpio L. Journal of Entomology and Zoology Studies 2017; 5(6): 1598-1602

Ashraf Abdel-Fattah Mostafa, Abdulaziz Abdulrahman Al-Askar, Mohamed Taha Yassin. 2020. Anti-saprolegnia potency of some plant extracts against Saprolegnia diclina, the causative agent of saprolengiasis, Saudi Journal of Biological Sciences, Volume 27, Issue 6, 2020, Pages 1482-1487, ISSN 1319-562X,
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Elgendy, M.Y., Ali, S.E., Abdelsalam, M. et al. Onion (Allium cepa) improves Nile tilapia (Oreochromis niloticus) resistance to saprolegniasis (Saprolegnia parasitica) and reduces immunosuppressive effects of cium. Aquacult Int (2022). https://doi.org/10.1007/s10499-022-01035-x

Fernández-Benéitez María José, Manuel Eloy Ortiz-Santaliestra, Miguel Lizana, Javier Diéguez-Uribeondo, Saprolegnia diclina: another species responsible for the emergent disease ‘Saprolegnia infections’ in amphibians, FEMS Microbiology Letters, Volume 279, Issue 1, February 2008, Pages 23–29, https://doi.org/10.1111/j.1574-6968.2007.01002.x

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