It looks like it might be Sapolegnia - me.
White or discolored 'cotton-wool' patches on the skin or fins. Often accompanied by inflammation and skin erosion
1) Saprolegnia (fungus) infection
2) Columnaris infection (mouth fungus/cotton wool disease).
Take sample for microscopic examination which would show typical long 'gliding' bacteria (Flexibacter sp).
Saprolegniasis
Cause
"Sap" is traditionally regarded as a secondary infection which infects the fish after another primary stress has affected the fish.
This can range from infection by a parasitic organism, high ammonia levels, low oxygen levels to stress in harvesting.
The only exception to this appears to be in times of severe change in water temperature (5 degrees plus) over a short period of time when water temperatures are below 16 degrees Celsius.
Consequently this "winter kill" is only seen in late autumn or early spring.
Saprolegnia lives on dead and rotting organic material.
As a preventative treatment it is therefore advisable to devise methods to ensure that you remove most of this material from your ponds.
Regular water exchanges at times of increased risk is one method.
PREVENTION
These diseases are extremely difficult to treat - no doubt due to the facts that:
a) they are essentially diseases of the internal organs i.e. spleen, brain, liver, etc., which are difficult to reach with medications, and,
b) apart from disease 6 (Saprolegnia), there are no easily-seen external symptoms until the disease is already well advanced.
The key to successful avoidance however is undoubtedly keep the system scrupulously clean at all times. Never overfeed, use the Nitrate, Nitrite, Ammonia, pH and Hardness test kits to ensure that water management, and water chemistry are always maintained within optimal limits.
