Flobibs

My red cap oranda has black on it's wen. I've had him for over 2 weeks now and the black appeared 4 days ago. He is eating and acting fine. Is in a 65 gallon tank with 2 other fancy goldfish bought the same day, same petstore. The others show no signs. tank cycled and water perfect: ph: 7,6 ; nitrite: 0; nitrate: 0; ammonia: 0; temp : 21 degrees celcius 60% water changes every 5 days Photo of goldfish

Test Results for the Following: * Ammonia Level : 0 * Nitrite Level : 0 * Nitrate level : 0 * Ammonia Level: 0 * Nitrite Level :0 * Nitrate level : 0 * Ph Level, Tank : 7,6 * Ph Level, Tap : 7,6 Other Required Info: * Brand of test-kit used and whether strips or drops? JBL strips * Water temperature?20 celcius * Tank size and how long has it been running? 65 gallons, running for 2 months * What is the name and "size of the filter"(s)? Fluval 360 canister * How often do you change the water and how much? 50 to 70 percents every 5 to 7 days. * How many days ago was the last water change and how much did you change? 4 days ago, 60 percent * How many fish in the tank and their size? 3 small fancies, 6 months old * What kind of water additives or conditioners? Tetra water conditionner * What do you feed your fish and how often? Main diet Saki Hikari Color inhance, small quatities, twice a day. Sometimes a third feeding of peas or spinach or lettuce. Brine shrimp, bloodworms... * Any new fish added to the tank? No * Any medications added to the tank? No

I've had my goldfish for 2 weeks now and they are all doing fine. I own one red cap oranda, one calico oranda and a black ranchu. Bought them all the same day and they all live in a 65 gallon tank. I do 60 percent water change every 6 to 7 days. The water is fine but I noticed today some black on my red caps wen ... do any of you guys no what it is ? Is it serious ? And how to treat it . Thanks