I have a large goldfish in a 4 foot glass tank who is 3 and a half years old, he has a red spot only on his tail fin, I don't think it is ancor worms though I have recently fed them live bloodworms and daphnia, he is feeding fine but seems to sit quieter than usual behind the plants, although he comes out as soon as I go near the tank, thinks its feeding time any one have any ideas what the spot could be.