I understand the importance of getting the water temperature close when transferring my goldfish from the tank back to the pond. I can do that as the water temp raises into the upper 60's. Also allowing the mixing of waters over a period of time.

My question is in the pond there will be a night/day difference of 15 degrees as far as I can tell. They tolerated this quite well last year but now they have been in stable temps, all winter.

At what night time temperatures would it be considered safe for transference? Has there been any studies done on this subject. The pond is about 30 inches deep, has foam filter, and circulation. The frogs are out in it, although they stay in untill it warms up. The plants are beginning to grow and one is actually blooming. I would love to see any Info. that you can suggest.
Thanks ahead of time.

I only bring my fancy goldfish inside for the winter (Oranda's and Pearlscale). The Koi, Comets, Shubunkins, Sarasa Comets and variations of them all stay outside all winter long with a deicer. I'll bring the fancies out when the the water temp consistantly stays around 60 or higher (even with the cold nights). It is probably okay to bring them out now as long as you don't feed them should the water temp dip below 55. Just make sure the water temps are about the same when you transfer them. You don't want to shock them.