Fish Fungus (Cotton Wool Disease): Spotting and Treating Saprolegnia

If you are looking at a tuft of white or greyish cotton-wool growth on a fish, you are almost certainly looking at Saprolegnia, the water mould hobbyists just call fungus. The first thing worth saying is that it rarely attacks a healthy fish out of nowhere. Fungus is an opportunist. It settles on tissue that is already damaged, dead or weakened, which means the fluffy growth is a symptom of an earlier problem as much as a disease in its own right. Treat only the fuzz and it tends to come back; find out why the fish was vulnerable and you usually stop it for good. I have dealt with it a fair few times over the years, most often on a fish that had been nipped, chilled or kept in water I had let slide, and the pattern is always the same.

What fungus actually is

What we call fungus in the aquarium is usually Saprolegnia, a genus of water mould. Strictly it is an oomycete rather than a true fungus, which sounds like pedantry but actually matters for treatment, because the antifungal drugs used in human or veterinary medicine do very little against it. What you see is a growth of fine threads, white to grey, sometimes picking up a greenish or brownish tinge as it traps debris and algae. On a fish it looks like a smear of wet cotton wool or a patch of fluff, raised off the body in three dimensions rather than lying flat.

It most often appears on a wound, a fin edge, the mouth, or anywhere the protective slime coat has been breached. Its spores are present in essentially every tank, all the time, doing no harm. They only get a foothold when they find tissue they can colonise, which is why fungus and injury or stress go hand in hand.

Is it actually fungus? Columnaris looks the same

Before you reach for a treatment, it is worth a hard look, because the single most common mistake is confusing true fungus with columnaris, and the two want opposite responses. Columnaris is a bacterial infection rather than a mould, and it can produce a pale, fuzzy-looking patch that fools people constantly. It is sometimes called cotton-mouth or cotton-wool disease too, which does not help.

A few things help me tell them apart:

• True fungus (Saprolegnia) forms distinct, raised tufts, that classic three-dimensional cotton-wool look, usually on a specific wound or spot. It tends to come on gradually and favours cooler water.
• Columnaris looks flatter and more like a film, a pale grey or yellowish patch, often around the mouth or along the back like a saddle. It can move fast, killing within a day or two in warm water, and it thrives in the heat rather than the cold.

If a pale patch appears suddenly and spreads quickly in a warm tank, especially around the mouth, treat it as a bacterial problem and not as fungus; an antifungal will not touch columnaris and you will lose time you do not have. When I genuinely cannot tell, I treat for the faster-moving, more dangerous of the two, which is the bacterial infection, and keep a close eye on how it responds. If you are in the wider “my fish are declining and I do not know why” situation, why are my fish dying walks through the broader diagnostics.

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What triggers it

Because fungus is secondary, the useful question is never “how do I kill the mould” but “what let it in”. In my experience the triggers are almost always one of these:

• Physical damage. A torn fin from fighting or netting, a graze from sharp decor, a bite. The slime coat is broken and the mould moves in. Shell-dwellers and other cichlids that squabble over territory are prime candidates after a scrap.
• Poor water quality. Ammonia or nitrite from an uncycled or crashed tank burns the gills and skin and knocks the immune system flat. A tank running dirty is a tank primed for fungus.
• Cold. Saprolegnia loves cooler water. A failed or undersized heater, or a fish kept below its comfortable range, raises the risk sharply.
• Stress. Bullying, bad shipping, a long spell of poor conditions. A stressed fish produces less protective slime and mounts a weaker defence.

Fix the trigger and you are doing most of the real work. Dosing a medication into water that still reads ammonia, or into a tank running at 20°C because the heater died, is treating the smoke and leaving the fire. Get a test kit on the water first; aquarium water parameters covers what to check and what the numbers mean.

How I treat it

For a mild, localised patch on an otherwise stable fish, I often do very little beyond fixing the cause and improving the conditions. Clean water, a sensible temperature and a well-fed fish will shrug off a small infection surprisingly often, because a recovered slime coat does the job the medication would.

When it needs more than that, this is the order I work in:

Sort the water and warmth first. A water change, confirmed-clean parameters and a temperature in the fish’s proper range are the foundation of every recovery. This step alone turns a lot of cases around.

Salt, for species that tolerate it. Aquarium salt is a gentle, effective first-line treatment against fungus and it helps the fish rebuild its slime coat. It is not for everyone, though; a number of soft-water and scaleless fish dislike it. I keep the Rift Lake and livebearer side of my fishroom in mind here, as those hard-water species generally take salt well, but I would be cautious with soft-water tetras, loaches and the like. I have written up doses and the cautions in using aquarium salt; treat the figures there as guides and err low for sensitive fish.

A proprietary antifungal, in a hospital tank. For a stubborn or spreading infection, a dedicated antifungal remedy is the next step. The traditional active ingredients work against water moulds where the human-medicine antifungals do not. Two important cautions: many of these treatments harm your biological filter and some are rough on scaleless fish and fry, so I run them in a separate hospital or quarantine tank rather than dosing the display. Follow the label dose for your tank volume exactly; work it out with the volume calculator if you are not certain how much water you are actually treating, because overdosing these is easy and dangerous.

Isolate badly affected fish. Moving a visibly infected fish to its own tank protects the others, stops you medicating the whole display, and lets you watch it closely. It also keeps your main tank’s filter out of harm’s way.

A note of sensible caution, because this involves live animals and medication: identify the problem as best you can before you dose, never combine treatments without knowing they are compatible, and if a fish is deteriorating fast, slow down and reassess rather than throwing a second chemical at it. More fish are lost to panicked over-treatment than to the original infection.

Fungus on eggs

This one is different and worth its own mention, because it comes up constantly for anyone breeding fish. Fungus loves fish eggs, particularly any egg that was infertile or has died, and it spreads from a dead egg to the healthy ones around it. A clutch can go from clear to a fuzzy white mass alarmingly quickly.

In the wild and in a well-run breeding setup, the parents handle this themselves. Cichlids, including the shell-dwellers I keep, fan their eggs constantly and pick out any that fungus has taken, which is one of several good reasons to let a stable pair raise their own clutch rather than stripping it. If you are hatching eggs artificially without the parents, the job falls to you: keep gentle water movement over the clutch, remove any egg that goes white the moment you spot it, and keep the water spotless. Methylene blue is the classic additive for protecting a clutch against fungus in a hatching container, but it stains everything it touches and wipes out beneficial bacteria, so it belongs in a bare hatching tub and never in your display. If your shell-dwellers keep producing eggs but you never see fry, fungus on the clutch can be part of the story; I cover the wider breeding troubleshooting in why won’t my shell-dwellers breed.

The short version

Cotton-wool fungus is a messenger. The white fluff is real and you should treat it, but it is telling you that a fish got injured, chilled, stressed or left in poor water, and that is the thing to put right if you do not want to see it again. Confirm it is actually fungus and not columnaris, clean up the water and warmth, use salt or a proprietary antifungal as the case needs in a hospital tank, and keep your eggs clean if you are breeding. Do that and most cases clear without drama. Keep dosing a fish in bad water and you will be back here in a fortnight.

As always, these are guides from my own tanks and from reputable aquarium sources, not veterinary prescriptions. Fish, species and situations vary; when in doubt, act on the conservative side and watch how the animal responds.