Overview
You net your goldfish for a routine inspection and notice a tuft of white fluff clinging to the edge of a torn fin, like a tiny cotton ball stuck to wet skin. True fungus, caused by Saprolegnia and related oomycete water molds, is an opportunistic infection that colonizes dead or damaged tissue on fish, producing the characteristic fluffy white or grey cotton-wool growths. Unlike bacterial infections such as Columnaris that attack living skin and spread as flat grey sheets, Saprolegnia is a secondary invader that requires compromised tissue to establish. It is one of the most misidentified conditions in aquariums because hobbyists see white growth and reach for antifungals without addressing the wound, bacterial infection, or water quality issue that opened the door. In my practice, I see true fungus most often in pond fish during autumn temperature drops, in tanks after fin rot outbreaks, and on eggs where unfertilized specimens become oomycete breeding grounds. The oomycete itself rarely kills a healthy fish, but it is a red flag that something else has already damaged the tissue.
Identification
Saprolegnia infection is an opportunistic oomycete disease characterized by fluffy, three-dimensional white or grey cotton-wool tufts that grow from dead or damaged tissue on the body, fins, mouth, or eyes. The growths are raised, filamentous, and easily distinguished from the flat, slimy patches of Columnaris or the discrete salt-grain spots of Ich. Fungal tufts often begin at the margin of a wound, fin tear, or ulcer and expand outward in irregular, branching patterns. Infected fish may show lethargy, reduced appetite, and clamped fins as the underlying condition progresses. On eggs, Saprolegnia appears as fuzzy white growths that spread rapidly from unfertilized eggs to adjacent viable ones. Do not confuse true fungus with bacterial infections: Columnaris produces smooth, grey-white lesions with a slimy texture and often forms a saddleback pattern. Physical injuries create clean wounds without the filamentous overgrowth. Ich presents as uniform, raised white spots distributed across the body, not localized fluffy tufts.
Life Cycle
Saprolegnia is an oomycete water mold with a life cycle that includes motile zoospores, non-motile cysts, and filamentous hyphae that colonize organic matter and compromised tissue in aquatic environments. The oomycete releases zoospores into the water column; these microscopic, biflagellated cells swim toward decaying matter, dead tissue, or stressed fish and attach to form germ tubes. Within 24–48 hours at 18–22°C/64–72°F, the germ tubes develop into branching hyphae that penetrate the substrate and produce the visible cotton-wool growths. Saprolegnia thrives in cool water between 10–20°C/50–68°F and in nutrient-rich conditions with high organic waste. Unlike Ich, which has a temperature-dependent cycle that accelerates with heat, Saprolegnia actually slows above 25°C/77°F, which is why raising temperature is sometimes used as a supportive measure. The oomycete can persist on decaying plant matter, uneaten food, and substrate debris for weeks, making substrate vacuuming and water changes essential for control.
Treatment
For precise medication dosing, use the Medication Dosage Calculator to calculate the amount based on your actual aquarium volume and product instructions.
Saprolegnia requires antifungal treatment targeting the visible hyphae combined with correction of the underlying tissue damage or water quality issue, because the oomycete is always secondary and will return if the primary problem persists. The most effective protocol I use begins with methylene blue at 3 mg per liter as a 30-minute bath for individual fish, or 1 mg per liter for prolonged treatment in a hospital tank or pond. Methylene blue is a thiazine dye with potent antifungal activity that also improves oxygen transport in fish with gill involvement. For tank-wide outbreaks, acriflavine-based products such as API Fungus Cure are effective at suppressing oomycete growth and preventing spread to other compromised fish. Malachite green at 0.1 mg per liter is highly effective but toxic to invertebrates and stains silicone sealant; use only in fish-only systems. Hydrogen peroxide dips at 3% concentration, 1–2 ml per liter for 5 minutes, can remove visible tufts but are risky and should be reserved for experienced keepers. This is the step most beginners skip, and it is why treatment fails: they dose antifungals into dirty water without removing the dead tissue or treating the bacterial infection that created the wound. Before any antifungal treatment, perform a large water change, vacuum the substrate to remove organic debris, and treat underlying bacterial infections with antibiotics such as kanamycin if fin rot or ulcers are present. Remove activated carbon during treatment. Fungal tufts on eggs can be treated with methylene blue at 1–2 drops per gallon, with prompt removal of unfertilized white eggs to prevent spread.
Prevention
Always isolate new or sick fish; see our guide on Quarantine Tanks: Essential Disease Prevention.
Preventing Saprolegnia means eliminating the dead tissue and poor conditions that allow zoospores to colonize fish. Maintain ammonia at 0 ppm, nitrite at 0 ppm, and nitrates below 20 ppm through regular water changes and adequate filtration. Avoid rough handling during netting or transport that causes scale loss or fin tears. Treat bacterial infections such as fin rot and Columnaris promptly before fungal secondary invasion occurs. In outdoor ponds, reduce organic debris by vacuuming the bottom and removing dead leaves before autumn temperature drops, when oomycete growth accelerates and fish immunity slows. During breeding, remove unfertilized eggs within 24 hours and maintain gentle aeration to prevent stagnant areas where the oomycete proliferates. Quarantine new fish for 14 days and inspect for wounds or fin damage that could become fungal targets. In my experience, the hobbyists who never see true fungus are the ones who maintain pristine water, handle fish gently, and treat the first sign of bacterial infection before Saprolegnia ever gets a foothold.