What are the symptoms of Saprolegniasis?

White to grey cotton-wool tufts of hyphae on the body, fins or mouth, with radiating edges; Visible wounds, redness or missing scales beneath the tufts; Weight loss, clamped fins, poor appetite, often resting on the bottom or in a corner; Lesions spreading from one site to several, with green or brown material caught in the tufts. It usually starts with: Body wounds from netting, transport or fighting give the mould an entry point; Reproduces noticeably faster at low temperatures, often below 15-20 °C; Deteriorating water and accumulated organic debris give the mould a substrate; Chronically stressed fish with lowered immunity, such as new arrivals or bullied individuals.

How do you treat Saprolegniasis?

In order: 1. Isolate the affected fish (Saprolegnia is a secondary infection, so move the fish to a hospital tank to stop other fish from pecking the wound, and check the display tank for aggressive tankmates.) 2. Find and fix the underlying cause (This is the decisive step: look for an unhealed wound, a low temperature, or elevated ammonia and nitrite. Killing the mould without fixing the cause guarantees a relapse after treatment.) 3. Change water and clear organic debris (Change 30-50% of the water with matched temperature, siphon out leftover food and waste, and rinse the filter pads. The mould is an opportunist, so clean water is itself part of the cure.) 4. Antifungal treatment in the hospital tank (Use methylene blue or malachite green as a bath at the label dose, in the hospital tank only. Methylene blue affects plants and the biofilter; malachite green carries high toxicity and residue risk, so avoid it when possible.) 5. Low-level salt bath as support (Use 1-3 g of non-iodised salt per litre as a short or hospital-tank bath, halving it for sensitive species. Scaleless fish such as loaches, plecos and corydoras do not tolerate salt - skip this step for them.) 6. Watch for a week for wound healing (The tufts fall away after treatment, but the real sign is new tissue closing the wound; if fresh hyphae appear within a week, the underlying cause is still there - go back and re-check water and temperature.)

What medication treats Saprolegniasis?

Methylene blue: Hospital tank only, as a bath at the label dose. It affects plants and nitrifying bacteria, so change water and run activated carbon afterwards instead of soaking the fish long term.; Malachite green: Hospital tank only, at the label dose. Toxicity and carcinogenicity concerns are significant and it is restricted in many regions; it also harms plants and the biofilter, so keep it as a last resort.; Povidone-iodine: Used for local wound care, diluted to the label and applied or bathed briefly with strict timing. It also affects invertebrates and the biofilter, so do not treat the whole display tank.

G-04 / 05Freshwater

Saprolegniasis水霉病

Moderate. The mould itself is rarely fatal, but it flags an unresolved wound or environmental problem, and treating only the mould leads to repeated outbreaks.

Illustration to come

G-04 / 05 · Saprolegniasis

SpecificationsDisease atlas

Cause: Fungus
Severity: Moderate
Contagion: Low contagion
Corals / inverts: Not safe
Also known as: Cotton wool disease、Water mould infection

Symptoms

White to grey cotton-wool tufts of hyphae on the body, fins or mouth, with radiating edges
Visible wounds, redness or missing scales beneath the tufts
Weight loss, clamped fins, poor appetite, often resting on the bottom or in a corner
Lesions spreading from one site to several, with green or brown material caught in the tufts

Typical triggers

— Body wounds from netting, transport or fighting give the mould an entry point
— Reproduces noticeably faster at low temperatures, often below 15-20 °C
— Deteriorating water and accumulated organic debris give the mould a substrate
— Chronically stressed fish with lowered immunity, such as new arrivals or bullied individuals

Treatment, in order

1 · IsolateIsolate the affected fishSaprolegnia is a secondary infection, so move the fish to a hospital tank to stop other fish from pecking the wound, and check the display tank for aggressive tankmates.
2 · Water changeFind and fix the underlying causeThis is the decisive step: look for an unhealed wound, a low temperature, or elevated ammonia and nitrite. Killing the mould without fixing the cause guarantees a relapse after treatment.
3 · Water changeChange water and clear organic debrisChange 30-50% of the water with matched temperature, siphon out leftover food and waste, and rinse the filter pads. The mould is an opportunist, so clean water is itself part of the cure.
4 · MedicationAntifungal treatment in the hospital tankUse methylene blue or malachite green as a bath at the label dose, in the hospital tank only. Methylene blue affects plants and the biofilter; malachite green carries high toxicity and residue risk, so avoid it when possible.
5 · SaltLow-level salt bath as supportUse 1-3 g of non-iodised salt per litre as a short or hospital-tank bath, halving it for sensitive species. Scaleless fish such as loaches, plecos and corydoras do not tolerate salt - skip this step for them.
6 · ObserveWatch for a week for wound healingThe tufts fall away after treatment, but the real sign is new tissue closing the wound; if fresh hyphae appear within a week, the underlying cause is still there - go back and re-check water and temperature.

The standard treatment uses copper or formalin, which is lethal to corals, shrimp, snails and everything on live rock. Treat in a quarantine tank only, and leave the display tank fallow for the full period above before returning the fish.

Medications and cautions

Methylene blue: Hospital tank only, as a bath at the label dose. It affects plants and nitrifying bacteria, so change water and run activated carbon afterwards instead of soaking the fish long term.
Malachite green: Hospital tank only, at the label dose. Toxicity and carcinogenicity concerns are significant and it is restricted in many regions; it also harms plants and the biofilter, so keep it as a last resort.
Povidone-iodine: Used for local wound care, diluted to the label and applied or bathed briefly with strict timing. It also affects invertebrates and the biofilter, so do not treat the whole display tank.

Prevention

Use a soft net when moving fish to avoid skin abrasions
In cold seasons keep the temperature inside the species range rather than at its lower limit
Quarantine new arrivals and isolate bullied individuals promptly
Change water and clean the substrate regularly to remove the organic substrate moulds feed on

Frequently asked questions

What are the symptoms of Saprolegniasis?: White to grey cotton-wool tufts of hyphae on the body, fins or mouth, with radiating edges; Visible wounds, redness or missing scales beneath the tufts; Weight loss, clamped fins, poor appetite, often resting on the bottom or in a corner; Lesions spreading from one site to several, with green or brown material caught in the tufts. It usually starts with: Body wounds from netting, transport or fighting give the mould an entry point; Reproduces noticeably faster at low temperatures, often below 15-20 °C; Deteriorating water and accumulated organic debris give the mould a substrate; Chronically stressed fish with lowered immunity, such as new arrivals or bullied individuals.
How do you treat Saprolegniasis?: In order: 1. Isolate the affected fish (Saprolegnia is a secondary infection, so move the fish to a hospital tank to stop other fish from pecking the wound, and check the display tank for aggressive tankmates.) 2. Find and fix the underlying cause (This is the decisive step: look for an unhealed wound, a low temperature, or elevated ammonia and nitrite. Killing the mould without fixing the cause guarantees a relapse after treatment.) 3. Change water and clear organic debris (Change 30-50% of the water with matched temperature, siphon out leftover food and waste, and rinse the filter pads. The mould is an opportunist, so clean water is itself part of the cure.) 4. Antifungal treatment in the hospital tank (Use methylene blue or malachite green as a bath at the label dose, in the hospital tank only. Methylene blue affects plants and the biofilter; malachite green carries high toxicity and residue risk, so avoid it when possible.) 5. Low-level salt bath as support (Use 1-3 g of non-iodised salt per litre as a short or hospital-tank bath, halving it for sensitive species. Scaleless fish such as loaches, plecos and corydoras do not tolerate salt - skip this step for them.) 6. Watch for a week for wound healing (The tufts fall away after treatment, but the real sign is new tissue closing the wound; if fresh hyphae appear within a week, the underlying cause is still there - go back and re-check water and temperature.)
Is Saprolegniasis contagious, and how urgent is it?: Contagion: low contagion. Severity: moderate. Cause: fungus. Moderate. The mould itself is rarely fatal, but it flags an unresolved wound or environmental problem, and treating only the mould leads to repeated outbreaks.
What medication treats Saprolegniasis?: Methylene blue: Hospital tank only, as a bath at the label dose. It affects plants and nitrifying bacteria, so change water and run activated carbon afterwards instead of soaking the fish long term.; Malachite green: Hospital tank only, at the label dose. Toxicity and carcinogenicity concerns are significant and it is restricted in many regions; it also harms plants and the biofilter, so keep it as a last resort.; Povidone-iodine: Used for local wound care, diluted to the label and applied or bathed briefly with strict timing. It also affects invertebrates and the biofilter, so do not treat the whole display tank.
Is the treatment for Saprolegniasis safe for corals and inverts?: No. The standard treatment (copper or formalin based) is lethal to corals, shrimp, snails and everything on live rock. Treat in a quarantine tank only, and leave the display tank fallow for the full period given above.