Hello, I woke up today and found one of my koi with a swollen tail. I suspect it might be gill flukes because I also saw him flashing a day before but I don't know if swollen tails are another effect of flukes. He is still active and eating as of now and the other fish (goldfish) seem to be healthy too. My water parameters are good (ammonia: 0 nitrite: 0 nitrate: ~20). Does anyone know what kind of disease this could be and how to treat it?
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