Someone should tell this to Freddy, my 8 1/2 inch ryukin/oranda mix. As soon as hit temp gets up as high as 70 he starts to lay around the bottom of the tank, the red veins appear in his fins (stress) and his breathing becomes labored. He is not the only fancy Goldfish
I have seen this same problem with... I have seen it over and over during the past 20+ yrs. There is nobody out there who will convince me differently because 20+ yrs of experience with Goldfish
of all types tells me that fancy Goldfish
do best in cold water, comets do better in cool water but can tolerate warmer temps for a period of time.
(and no, there is no issue with the filtration in Freddy's tank, nor is there an issue with bacteria colonies at the lower temp)
In the winter months here, Freddy's tank temp goes as low as 50F, and this tends to be when he is the most active and healthiest. During summer months it can be a challenge to keep his water temp cool enough to keep him happy and active.
I have also noticed feeding differences at the different temps, and he (along with many others) tend to feed better when the temp is colder.
If Lupin disagrees with me then we'll simply have to agree to disagree.