When I went in to my local pet shop the other day was actually to get some food for my hamster I asked the guy if there was any way that you could tell the difference between a male and a female common fantail goldfish and he said that you couldn't tell the difference. On getting home i decided that I asked one of my mates who keeps goldfish herself and she said that you could tell by the shape of the tail. I'm just wondering whether this is fact or fiction and is there any obvious way to tell the difference between a male and a female?