Can you post any clear photos of the sick fish? Also can you tell us about your tank, water parameters for ammonia, nitrite, nitrate, and pH, tank size, what other fish are in the tank and how many of each species? Water change/maintenance schedule? (how much, how often?) type of substrate? water temp? type of filtration? filter media? live plants in the tank? foods you are offering, what kind, how much and how often?
The more information you can offer the faster we can help. This sounds like a fungal infection, but a photo would help a lot to assure an accurate diagnosis. If there are any other symptoms, please list those too... loss of appetite, red veining in the fins, color change, change in activity level/behavior, sores, etc.? What kind of
Goldfish is this? comet or fancy?
I'll keep watch on this thread and do what I can to help you.
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