Causative agent: Saprolegnia spp. / Achlya spp. — water moulds (oomycetes)
Fungal infections in freshwater fish, most commonly caused by the water mould Saprolegnia, typically appear as white or greyish cotton-wool-like tufts growing from the fish's skin, fins, or around injuries. Despite their name, Saprolegnia infections are caused by oomycetes (water moulds) rather than true fungi, though antifungal medications are effective against them. In Australian aquariums, Saprolegnia most commonly strikes fish that are already weakened by injury, bacterial infection, cold temperatures, or stress — acting as a secondary opportunist. Fish breeders in Australia frequently encounter Saprolegnia on eggs in breeding tanks, where it can quickly spread and kill an entire clutch. Treatment with Aristopet Fungus Cure or Aquasonic Fungonex, combined with improving water temperature and quality, is typically effective when caught early. Addressing the underlying cause — be it a wound, bacterial infection, or poor water conditions — is essential to prevent recurrence.
Symptoms
- •White or grey cotton-like tufts growing on skin, fins, or mouth
- •affected area becomes red and inflamed underneath
- •may follow an injury or bacterial infection
- •fish may appear lethargic
Treatment
- •Antifungal medications
- •malachite green
- •methylene blue
- •salt baths
- •improve water conditions
- •treat underlying bacterial infection if present
Australian Medications
- •Aristopet Fungus Cure
- •Aquasonic Fungonex
- •Aquasonic Methylene Blue
Commonly Affected Fish
- •All freshwater fish
- •fish eggs especially vulnerable to Saprolegnia
Important Notes
Not a true fungus — it is a water mould (oomycete). Antifungal medications still work. Frequently secondary to bacterial infection or injury.