Kai: These are my Japanese Fantail gold fish. I have kept Fantails before, but never tried to breed them. I’ve heard double tail varieties are more challenging to breed than single tails, so this will be an interesting experience.

If one is breeding goldfish without artificial heating, it is critical the fish are nutritionally conditioned before Winter, when they go into a metabolic stasis. You will have very little time to prepare them in the Spring, because spawning begins almost immediately, to which the females are using up what’s left of their Winter fat reserves to produce eggs. And they will do this whether you intend to breed them or not.
If they are not prepared before hand, they may not breed come Spring, or at least, may produce scantly, and can even become egg-bound. Egg-binding is not as dangerous in fish as it is in birds, but you can still lose fish to this if they are not prepared to spend the nutrition.
In general, it’s not good for the fish to go into the cold months without preparation for its own personal health as well. I lost a fantail to this very problem recently.

Although these stout, double tailed fish I acquired, I also got a pleasant surprise last year in a batch of shubunkin fry I raised – several double tailed individuals hatched, though only one survived. It was really quite charming and unexpected, but the survivor has turned out to be quite a beautiful fish. I do not know how she ended up with a split tail, but it is quite fascinating!