They are a cool-water species. They do not need a heater unless the room they are in is abnormally cool. Putting a heater in if the tank stays below 70 is not a bad idea. It is not good to push fancier varieties below 50F, some say 60F. It is too stressful for them. Obviously at least some can tolerate it, but there is no need to push the limits of their life.
All goldfish are the same species. There are different breeds, just like with dogs, but they are all still Carassius auratus. The fancier varieties are not as hardy. So a lionhead is not as hardy as a regular old fantail, which is not as hardy as a plain old goldfish. (Comet refers to a regular goldfish, but one with longer fins, especially the caudal fin.) So not all goldfish are as hardy as one another.
Ranchus are not the Japanese version of a lionhead, it is a different breed. Ranchus have a few differences, the most obvious being an approximate 90 degree angle between the downward curving caudal peduncle and the caudal fin, whereas the lionhead's is almost straight, following the contour of the back.
All goldfish are the same species. There are different breeds, just like with dogs, but they are all still Carassius auratus. The fancier varieties are not as hardy. So a lionhead is not as hardy as a regular old fantail, which is not as hardy as a plain old goldfish. (Comet refers to a regular goldfish, but one with longer fins, especially the caudal fin.) So not all goldfish are as hardy as one another.
Ranchus are not the Japanese version of a lionhead, it is a different breed. Ranchus have a few differences, the most obvious being an approximate 90 degree angle between the downward curving caudal peduncle and the caudal fin, whereas the lionhead's is almost straight, following the contour of the back.