This summer i kept a handfull of my goldfish eggs in a separate tank just to rear a few for a friend, they are about 3 weeks old now and i have just noticed that 1 of them has 2 tails like a butterfly tail, i have never had fancy tailed fish in the pond so both parents are definitely single tail, is this normal? It is swimming about and feeding ok, hopefully if it keeps going i will keep that one myself .