The freeze resistant power of Goldfish in a pond is simply amazing
They hunker down as deep as they can, and if they find a pocket of unfrozen water, that is where they wait it out. What do they do while they wait – count fish, maybe?
It must be a long winter game of “how many of us have survived so far.” And it’s hard to tell who is alive, because they are so still. Belly up and encased in ice are sure signs.
There are ways to make winter more pleasant for Goldfish and a little help may be necessary to keep Koi healthy through the cold times.
Remove leaves and other organic matter from the pond.
The process of decomposition uses oxygen and produces small amounts of hydrogen sulfide, a gas that can be toxic to fish. While the gases do not normally reach harmful levels, the extra stress of winter, with decreased oxygen due to a frozen (or partly frozen) pond, can tip the balance. Fish and other aquatic creatures are especially sensitive to poor water quality in winter (as if freezing temperatures were not enough – now toxic gas – ahhhh).
A deep pond is better to escape the deep-freeze. Give them a bare minimum of 18 inches. Goldfish and Koi prefer 3 to 4 feet, respectively.
Try to keep some openings in the surface of the pond for oxygen/gas exchange. This may be difficult in some climates, but de-icers are available to keep a spot open. Moving water helps, so keep falls or fountains running as long as possible.
In extreme climates, a last resort may be to bring fish inside: perhaps to a basement. Keep in mind that the moves are stressful for them – in and out. If you feel you must bring them in, invest in large tanks with plenty of aeration.