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Back on 11/9/10 I bought 2 fairly small orange oranda's and put them in a 20 long by themselves with no gravel, no other tankmates, and an Odessy over the back filter. Last week we got what look like two koi as a gift to go with them. Don't know if there's a link there, but bottom line, the tail fins of both the oranda's has turned black. I'm new to goldfish, so I have no clue as to what that is. They seem healthy and happy, but I need help in determining what the black fins are about.