I understand the netting idea (thanks!), but why would I need a heater? I've never had one before, that's why I'm asking.
Thanks for the help!
Because unless the pond is of at least 3ft depth, a freeze during the winter could finish your goldfish off. The more depth a pond has, the more stable is temp will be and easier it will be for the goldfish to hibernate successfully and uninterupted through the winter.
If the temp warms up too much after a freeze during a short warming period in the winter, the goldfish can be fooled out of thier dormant state and think it is spring, which can be very bad for them if it is still actually winter and the freeze returns as the stress of coming out of hibernation and becomming active only to be forced back into hibernation can be too much for some goldfish to cope with. Parasites are also more active in warmer temps, and a short warming period in the winter can also make any parasites in the water active which can be very bad for any sleepy goldfish in the pond as their imune systems will not be as active as they would be during warmer months.
A deep pond of 3ft depth or more, or a heated pond (because a heater will help keep much more stable temps in the pond), is the best way to ensure the winter does not kill the goldfish off and that they come out of hibernation in spring healthy and untroubled
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If your goldfish are of fancy breeds then they cannot tolerate freezing temps at all, so an outdoor pond would always need to be kept above freezing temp levels (i would advise at least 11 degree's warm).
If they are breeds like comet or common goldfish, these slim type goldfish can tolerate freezing temps (although the pond would be too shallow for them if it was unheated), but will need a pond larger than 30gals (minimum recommend amount of gallons for a comet or common goldfish is 75gals, if you have 2 or more i would advise at least 90gals as these fish grow larger than fancy breeds as they can grow to over 12inches long and are also a lot more active).
Edit: ah you have fancy goldfish then- personally i would advise a heated pond of 40gals or more then (as fancy goldfish can't survive freezing temps) as generally speaking, with fancy goldfish stocking it is advised you have 20gals for the first fancy and 10gal for every one after that
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