Goldfish are cold blooded creatures. The speed of their metabolism depends upon the temperature of the water they are in. They are essentially Carp and so they have evolved to perform and do best in temperate conditions. Their best temperature range is from around 50F up to 70F. At 70F their metabolism is going full pelt and this is the temperature they breed at. Any warmer than around 73F and they can suffer burn out and shortage of breath because their gills are not efficient enough to collect sufficient oxygen from the oxygen depeted warm water. Below 50F their internal organs begin to slow down as the fish too slows down. They eat less and do less until the water gets cold enough for them to stop doing anything . They can then survive just above freezing (32F) by going into a kind of trance 'hibernation'.