Saprolegnia (Cotton Mold)

Put a flashlight next to your tank glass and look closely. Saprolegnia usually reveals itself in the details a casual glance misses.

Below is everything I wish I’d known the first time I dealt with this — written for keepers, not researchers.

Before going deeper, here’s the disease in one block:

• Pathogen. Saprolegnia parasitica (and other Saprolegnia spp.) (Oomycete)
• Typical hosts. All freshwater fish and eggs; particularly common as secondary infection
• Reported distribution. Worldwide in fresh water
• Temperature window. Outbreaks usually at 32–64 °F (0–18 °C); sometimes higher under stress
• WOAH-listed (notifiable). No
• Reference image datasets. CuraPeces (Fungi), Roboflow Eki (Saprolegniasis)

Background on the agent

Saprolegniasis is caused by Saprolegnia parasitica (and other Saprolegnia spp.), a oomycete in the Saprolegniales.

The biology is worth a minute of your time: it explains why the treatment that works for one disease often does nothing against another that looks identical at a glance.

Despite the cotton-like appearance, this is technically not a true fungus in some cases — oomycetes are water-molds, more closely related to algae than to mushroom-forming fungi. Practically, it doesn’t matter for treatment, but it does mean some antifungal medications designed for terrestrial fungi don’t work well in fish systems. Stick to treatments specifically labeled for aquatic use.

Susceptible species: primarily All freshwater fish and eggs. Different species within the same family show different vulnerability — even closely related fish can have wildly different clinical outcomes from the same pathogen. If you keep mixed species, observe each one independently; the apparently-healthy one may simply be a quieter carrier.

Where it comes from

Routes of transmission are the leverage point for prevention:

Ubiquitous zoospores in water; infects damaged or stressed fish, dead eggs, or fish in chilled/poor water.

Practical takeaway: nothing that touches an infected tank should touch a healthy one without disinfection. That includes your forearms.

Visual signs — what the disease looks like

Symptoms tend to appear in a fairly consistent order, even if the timing varies. Watch for:

• Cotton-wool-like white or grey patches on skin, fins or eggs, often around injuries or near the head. Tufts of fine fibers on the skin, mouth, or fins. Looks like a tiny clump of cotton.
• Underlying skin reddened. A change worth noting and timing — write down when it first appeared.

How to actually observe — most keepers skip this:

Stand in front of the tank for a full 5 minutes without doing anything. The fish will normalize to your presence in about 90 seconds. Most diseases declare themselves in the behavioral subtleties you only catch in those last 3 minutes — the small posture shifts, the unusual hovering, the slightly-asymmetric swim. Glance-and-go checks miss everything but the most florid signs.

Pair what you see on the fish with what’s happened in the tank lately. The symptom + recent history together is a much stronger signal than the symptom alone.

What to capture with your phone

The phone-photo trick: don’t trust your memory of what the fish looked like. Take a series of shots once a day. Disease progression often hides in a comparison you couldn’t do from recall.

What to capture:

• A wide shot of the whole fish from the side (use a black background card behind the glass if you have one).
• A tight macro shot of any visible lesion, fin damage, or color anomaly.
• A shot of the gills if you can briefly lift an operculum.
• A shot of behavior — yes, video is fine. Lethargy and erratic swimming are diagnostic in their own right.

Even a basic phone camera with steady hands and good light beats a fancy DSLR on autofocus through agitated water. Patience over equipment.

The environmental side of the equation

Most outbreaks have a context. Run through this list before you blame bad luck:

• New fish in the last 4 weeks. The single most common trigger, by a margin. If you skipped quarantine, this is your most likely vector.
• Recent temperature swing. A 3 °C / 5 °F change in either direction stresses the immune system enough to enable opportunistic pathogens.
• Ammonia or nitrite reading above zero. Even brief spikes damage gill tissue and open the door for secondary infection.
• Filter maintenance event. A full filter clean (vs. a rinse) can crash the bio-load briefly. Watch the next 48 hours closely.
• Aggressive tankmate. Bite wounds and abrasions are infection sites. The ‘pretty cichlid’ becomes the disease vector when a flank wound stops healing.
• Overstocking creeping up. Each new fish raises ambient pathogen load and competition for oxygen. Disease load rises non-linearly with stocking density.

For this specific disease, temperature matters: outbreaks concentrate at Outbreaks usually at 32–64 °F (0–18 °C); sometimes higher under stress. If your system runs through that band seasonally, raise vigilance during those weeks. Pond keepers in temperate climates should bookmark this — spring and autumn are when problems land.

First-response steps

Step by step, in the order that matters:

Isolate the affected fish. A bare-bottom hospital tank — heater, sponge filter pre-cycled, no substrate, no decor — lets you medicate at proper dosing without nuking the display’s biofilter or invertebrates. The hospital tank needs to match the display in temperature and parameters; sudden changes are an additional stressor the sick fish can’t afford.

Confirm the diagnosis before you medicate. Photo the symptoms (see the photo tip section), compare against reference images, and run the case through the Symptom Checker. Picking the wrong treatment class is worse than waiting an extra 12 hours to confirm. Don’t medicate blind.

Specific treatment. Salt baths (3 g/L for hours, or 10 g/L short bath), formalin, hydrogen peroxide on eggs; address underlying stress and injuries; remove dead material

Test the water on the source tank. Even after moving the fish out, your display almost certainly has an underlying parameter issue that enabled the disease in the first place. Test ammonia, nitrite, nitrate, pH, and (where applicable) salinity, KH, and temperature. Fix what you find before reintroducing anything.

Do a 25% water change on the display. Not 50% — that’s a parameter shock to the remaining fish. 25% with temperature-matched, dechlorinated water, then test again 12 hours later.

Quarantine new fish for a minimum of 4 weeks. During an active outbreak, don’t add anything to the system. Adding a new fish into a sick tank is committing it to the same exposure with zero protection.

Track everything. A simple notebook entry per day — date, observations, treatment dose, water-test numbers — pays off if you need to consult a vet or if the same issue recurs in 6 months. Memory is unreliable here; written records aren’t.

For the keeper reading this with a problem in front of them right now: don’t medicate in the next 15 minutes. Spend that 15 minutes on observation and water testing. The diagnosis you’ll arrive at from the cooler analysis will be better than the one you’ll commit to under stress. Two hours of right treatment beats two days of wrong.

Timeline and expectations

Healing follows a predictable pattern. Here’s roughly what you should see:

• First 48 hours: Stabilization. The fish stops getting worse. Often no visible improvement yet.
• Days 3–7: The most visible improvement phase. Behavior normalizes, eating returns, lesions begin to contract.
• Weeks 2–3: Tissue rebuilds. Fins regenerate from the base outward. Scales return over 4–6 weeks for most species.
• Beyond: A 30-day post-treatment observation window before reintroducing tankmates or adding new fish. Some pathogens persist subclinically.

Take a weekly photo. Recovery is easy to miss if you’re seeing the fish every day; comparison shots make progress (or stalling) obvious.

Avoiding a repeat

Long-term prevention comes down to a small set of repeatable habits:

Quarantine new arrivals for at least 4 weeks. This is the single biggest lever you have. Most introductions of disease into established tanks come from un-quarantined new fish or live plants/decor with adherent water. A separate 10-gallon tank with a sponge filter is sufficient for most species and costs less than one decent fish.

Stable water parameters. Test weekly even when the tank looks fine. Ammonia and nitrite at zero, nitrate under control (under 20 ppm for sensitive species, under 40 ppm for hardier ones), pH consistent across tests. Most outbreaks follow a measurable water-quality slip the keeper didn’t catch — not because the parameters are bad in absolute terms but because they shifted enough to stress the fish.

Don’t overstock. Crowding raises ambient pathogen load and lowers individual immunity. The most disease-free tanks I’ve seen are the ones that look slightly empty. A rule of thumb worth more than the standard ‘inch per gallon’ is to stock for half the bioload your filter is rated for, leaving headroom for water-quality buffering.

Equipment hygiene. Dedicated nets, siphons, and buckets per tank. If you must share, disinfect with a bleach dip (1:19 with water, 60 seconds, thorough rinse, then a 24-hour air dry). Color-coded gear per tank is a 5-dollar fix for what otherwise becomes a recurring problem.

Watch the fish daily. Two minutes per tank, every morning, with the lights on full. You catch the first hour of trouble that way — and the first hour matters. Build it into a routine you can’t skip; with the coffee, with the dog walk, whatever sticks.

Source matters. Buy from sellers who actively quarantine their incoming stock. The premium price reflects fewer disease introductions downstream. A fish that’s 30% more expensive but doesn’t bring in pathogens has saved you more than the markup.

Don’t ignore live plants and decor as vectors. Snails, tubifex worms, and any wet surface from another system carries water and the microbes in it. Rinse new plants thoroughly; consider a hydrogen peroxide dip (3% solution for 30 seconds, then rinse) for plants from unknown sources.

Mind the temperature range. Outbreaks are concentrated at Outbreaks usually at 32–64 °F (0–18 °C); sometimes higher under stress. If your system runs there seasonally, raise vigilance during those weeks. For pond keepers, this often means heightened spring and autumn watching when water temperatures pass through the danger band twice a year.

A note on look-alikes

If two diseases come up as candidates and you can’t tell them apart, treatment overlap is rare — picking wrong means a wasted week. Use the Symptom Checker to narrow it down, or check the photo galleries on the related pages below.

There’s no shortcut on this. There’s just paying attention and acting on what you see.

You may also want to read

• Branchiomycosis — for keepers narrowing down between this and a look-alike condition.
• Epizootic Ulcerative Syndrome — for keepers narrowing down between this and a look-alike condition.

Source

Primary reference: Noga, E.J. (2010) Fish Disease, 2nd ed.; Australia DAFF Field Guide ‘Other diseases of finfish’ chapters.

Read the full source: https://universe.roboflow.com/eki/fish-disease-t6b03

Editorial review by the Fishy Farmacy team. Last reviewed: May 2026.