Fungus (Saprolegnia)

Not actually a fungus

Saprolegnia is an oomycete, or "water mould" — a group more closely related to algae and to plant pathogens such as Phytophthora than to true fungi, despite the cottony, mould-like appearance. The distinction is not pedantry: oomycetes differ from true fungi at the cellular level, which is why treatments developed for fungal infections do not always work on them, and partly why fish "fungus" can be stubborn. In the wild and the aquarium it lives mostly as a saprotroph on dead organic matter, spreading by releasing motile zoospores that settle onto new surfaces — including vulnerable tissue on a weakened fish.

What it looks like — and what it is not

Saprolegnia appears as fluffy, cotton-like growths — white, grey or tan, sometimes tinted by trapped debris — on the skin, fins, mouth or eyes, almost always centred on an existing injury or lesion. Lifted from the water, the tufts collapse into a slimy film, which distinguishes them from the salt-grain spots of ich or the fine dust of velvet.

The most important confusion to avoid is with columnaris (Flavobacterium columnare), a bacterium that also produces pale, cottony patches — often called "mouth fungus." Columnaris spreads faster than Saprolegnia, especially at higher temperatures (it accelerates above 75 °F, where Saprolegnia slows), tends to produce a yellowish or greenish tinge, and characteristically attacks the saddle-back region near the dorsal fin as well as the mouth. Temperature context is a quick first screen: a cottonball patch appearing in a cold tank (below 64 °F) on a chilled fish is almost certainly Saprolegnia; a rapidly advancing pale lesion on a warm-water fish with mouth involvement leans toward columnaris. Treating empirically for fungus when the real problem is columnaris wastes critical time — the two require different agents.

For the advanced keeper: a wet-mount skin or gill scrape will resolve ambiguity. Saprolegnia hyphae are broad, non-septate (coenocytic), and clearly branching — visible at 100–400× as long, transparent, filamentous tubes. Columnaris produces characteristic 'hay-stack' bacterial columns or gliding filaments that are much finer and do not branch the same way. Velvet (Piscinoodinium) and ich (Ichthyophthirius) produce round or ovoid parasitic cells, not hyphae. If lesions do not respond to antifungal treatment within a week, a scrape is the next step before adding another chemical to the tank.

• Cottony white-to-grey tufts on skin, fins, mouth or eyes
• Often centred on a wound or an existing lesion
• Collapses to a slimy film out of water — not the powder of velvet or the grains of ich
• Columnaris key tells: faster spread, yellowish tinge, mouth/saddle-back location, warm-water context
• Microscopy: Saprolegnia hyphae are broad, non-septate, branching at 100–400×

Why it takes hold

Because Saprolegnia only colonises compromised tissue, an outbreak is a flag to look for an underlying problem: physical injury from handling or décor, fin rot or another infection that has already broken the skin, poor water quality, or — classically — a temperature drop. Saprolegniosis is so strongly associated with cold that aquaculture has long called it "winter fungus," striking pond and farmed fish when falling temperatures depress immunity while the mould keeps growing. The same logic applies in the aquarium to any chilled or recently stressed fish. Treat the tuft without addressing the cause and it returns, because the door that let it in is still open.

The egg problem

Saprolegnia is the bane of the breeding tank. It readily colonises dead and unfertilised eggs, fuzzing them over — and from that foothold its spores can spread to the healthy, developing eggs nearby, so a single patch of bad eggs can threaten an entire clutch. This is why breeders remove dead eggs promptly, keep spawning water pristine, and often use antifungal agents suited to eggs. The principle mirrors the situation on a fish: deny the mould its dead-organic-matter foothold and it cannot establish.

The treatment toolkit — principles and contraindications

Start with the environment and the wound: restore clean, stable water and temperature, remove any obvious source of injury, and clear dead material — spoiled eggs, uneaten food — that feeds the mould. For a localised patch on one fish, a hospital tank lets you treat without dosing the display, and is strongly preferred for any community or mixed tank.

Salt (sodium chloride) is a widely used first-line agent for Saprolegnia. It is worth knowing that scaleless fish — loaches (Cobitidae, Botiidae), many catfish (Corydoras, Ancistrus, Synodontis), eels, and knifefish — are significantly more sensitive to salt than scaled fish, and many small tetras, rasboras, and other soft-water species have limited tolerance as well. Salt also harms live plants and is lethal to freshwater invertebrates (snails, shrimp, crayfish). In a planted community tank with inverts, even a seemingly mild salt treatment can have significant collateral costs.

Formalin (formaldehyde solution) is an effective anti-oomycete agent used in aquaculture but carries its own risks: it consumes dissolved oxygen, making aeration essential during any treatment, and it is hard on biological filtration. Copper-based medications, broadly antifungal and antiparasitic, wipe out invertebrates and should never be used in tanks with shrimp, snails, or crayfish; they are also poorly tolerated by scaleless fish and may stain silicone.

Where a secondary bacterial infection is present alongside the Saprolegnia — common when the underlying wound is infected — antibiotics may be warranted, but antibiotic treatment can crash the nitrogen cycle by killing nitrifying bacteria. Treating in a hospital tank with established filter media, or having ammonia-sequestrant on hand, is prudent.

Because Saprolegnia is an oomycete, some broad-spectrum antifungal agents developed against true fungi have limited efficacy. Agent selection should follow current aquatic veterinary guidance. Shotgunning multiple OTC products in sequence — especially without a confirmed diagnosis — applies chemical stress without resolving the cause, risks breeding tolerant strains, and delays the effective treatment.

For prognosis: a small, clearly localised patch on a robust, otherwise healthy fish caught early carries a good prognosis when the underlying cause is addressed. Deep systemic involvement — extensive skin loss, the mould penetrating into muscle or the eye — is much harder to turn around. A fish that has lost a significant portion of its slime coat and skin to the mould, or one in which the triggering immunosuppression cannot be resolved (e.g. a terminal internal disease), realistically may not recover. An honest assessment early is kinder than prolonged, escalating treatment.

• Hospital tank strongly preferred — avoids dosing the display and protects inverts
• Salt: effective, but poorly tolerated by scaleless fish, small soft-water tetras, live plants, and all freshwater invertebrates
• Formalin: effective against oomycetes but drops dissolved oxygen — run extra aeration
• Copper: kills inverts; avoid in shrimp/snail tanks and with scaleless species
• Antibiotics (if secondary bacterial infection present): can crash biological filtration
• Shotgunning OTC products risks chemical stress and treatment resistance without resolving the cause
• Prognosis: good for small, early lesions on healthy fish; guarded to poor for deep/systemic involvement or unresolvable underlying cause

Biosecurity and quarantine

Saprolegnia zoospores are ubiquitous — they are in virtually every established aquarium and most water sources. Quarantine will not keep the organism out, but it will keep a stressed, injured, or immunocompromised new fish from seeding spores into a tank with fish that cannot afford the exposure. A standard four-week quarantine also catches the secondary infections (parasites, bacterial disease) that are the likely triggers for any subsequent fungal outbreak. Any fish showing visible Saprolegnia lesions should be moved to a hospital tank rather than treated in the display; this removes the spore load from the main tank and allows targeted treatment without chemical stress on healthy tankmates.

When Saprolegnia breaks out on multiple fish in the same tank, treating the display — not just individual fish — is warranted. Deep-clean substrate and décor; remove organic debris that feeds the mould; address the water quality failure that allowed a widespread outbreak. If the outbreak follows a new introduction, review the quarantine failure.

When to escalate: if lesions do not respond to correct treatment within seven to ten days, or if diagnosis is genuinely uncertain, a wet-mount skin scrape is the next step. A board-certified aquatic veterinarian or a diagnostic fish lab can culture the organism and, in complex cases, run sensitivity testing to guide agent selection. This is not overcaution — it avoids weeks of ineffective empirical treatment in a tank already under chemical stress.

• Saprolegnia is already present in most tanks — quarantine targets the vulnerable fish, not the organism
• Visible lesions: move to hospital tank to remove the spore load from the display
• Multi-fish outbreak: treat the system; deep-clean substrate and décor; address the root water quality problem
• No response in 7–10 days: wet-mount scrape or aquatic-vet consult before escalating medication

Prevention

Prevention is the standard husbandry checklist, read through the lens of injury and chilling: keep water quality high and temperature stable — cold snaps are the classic trigger — handle fish gently to avoid the wounds that give the mould its opening, and treat other infections before they break the skin. In the breeding tank, remove dead eggs and keep the water immaculate. A healthy, unbroken fish in clean, stable water offers Saprolegnia nothing to colonise.

• Keep water quality high and temperature stable — avoid chilling
• Handle fish gently; treat skin-breaking infections early
• Remove dead eggs and debris in breeding tanks
• An unbroken, healthy fish in clean water resists it