I don't change water that often in my ponds, just do top offs. That is why I mostly have a very light stocking of goldfish in them. I don't see temp swings by changing water to have that much effect on tropicals as the water will soon return to the "regular" temp by the next day when temps are high.
As inexpensive as this seller has these solar fountains, it is definitely worth having for a stock tank just to provide that bit of extra oxygen.
We haven't had an ice storm since I've started doing the coldframes but I do keep a pump in all the ponds during the winter to help prevent freezing. I do have tropical losses because they didn't tuber, even some of my favorites in the partially in ground pond last winter which should have stayed warmer than some of the above ground ponds. The one problem with doing the coldframes is that those ponds aren't getting rainwater to top them off & to freshen the water. I don't have enough space for rain barrels as that would save water for me for topping off/water changes.
