Saprolegnia: The Fungus That Only Attacks Weakness

Saprolegnia is genuinely everywhere, its spores exist in essentially every aquarium and every body of freshwater, floating invisibly until they find something to colonise. In a healthy fish with intact skin, this rarely matters. Saprolegnia is an opportunist in the truest sense: it needs damaged or dead tissue to get a foothold, which is why it shows up almost exclusively on existing wounds, already-diseased skin, or unfertilised fish eggs.

Where Saprolegnia actually shows up

• On fish eggs, particularly unfertilised or already dying eggs in a breeding setup, where it can then spread to healthy eggs nearby if not caught early
• On an existing wound or injury, a fin nip, a scrape against decor, or the site of a previous infection
• On a fish already weakened by stress, poor water quality, or another illness, where its general immune resistance is already lower than normal

What it looks like

Saprolegnia typically appears as a looser, genuinely fuzzy white or greyish growth, closer to cotton wool or a tuft of cotton than a flat patch, usually confined to one localised area rather than spreading broadly across the body the way a bacterial infection tends to. On fish eggs, an infected egg turns visibly white and fuzzy while healthy eggs nearby stay a clear, translucent colour, an important distinction for anyone breeding fish at home, since it is easy to mistake a healthy opaque egg for a fungal one.

Treating Saprolegnia on eggs

Infected eggs should be removed promptly with a pipette or soft forceps to prevent the fungus spreading to healthy eggs nearby. A prophylactic treatment of the whole clutch with methylene blue, at the dose stated on the product, is standard practice among breeders and meaningfully reduces losses even before any infection appears.

Treating Saprolegnia on adult fish

• Address the underlying wound or stressor first, since the fungus will simply return if the original vulnerability is not resolved
• A methylene blue or malachite green based treatment, dosed at the same strength as a standard ich treatment, is effective against the fungus itself
• Improve water quality with a partial water change, since Saprolegnia establishes far more easily in tanks with poor conditions and excess organic waste

The uncomfortable truth about underlying causes

A visible Saprolegnia patch on an adult fish is very often a secondary problem sitting on top of an underlying bacterial infection that is harder to see and harder to treat. Clearing the visible fungus without also addressing a possible bacterial cause underneath, sometimes with a course of antibiotic food if the fish is still eating, means the fungus can simply return. If a patch keeps recurring in the same spot despite antifungal treatment, an underlying infection is worth suspecting and treating alongside it.

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