Question
QUESTION: My husband and I live in zone 5 (Okanagan BC)and have goldfish in our 12x8x4(feet deep)pond.  Not knowing any better, last year we let the pond freeze over and the goldfish all survived. Now, having done a little investigation, it appears we should leave a hole in the surface for air exchange.  I'm curious, should we just leave the pond (and fish) to nature again or perhaps would inserting a PVC tube deep into the pond help keep the air exchange open?  How important is this air exchange? Thank you for your answer.

ANSWER: Hello Noelle!
Glad to hear that your fish made it through the winter last year. Goldfish can be quite hardy.
There is no guarantee that the fish will make it again without a de-icer and/or air pump.
There are too many outside influences that can impact the fish.
As they get bigger they emit more carbon dioxide, for example. This will gradually poison the pond for them if there is no oxygen exchange. There can also be some stratification of the water if there is a warm spell some time during the winter and the resulting fast temperature changes in the different layers of the water can harm and even kill them.
General accepted wisdom for Goldfish is to provide some means for oxygen to get into the pond.
Koi and other pond fish have more stringent requirements.

---------- FOLLOW-UP ----------

QUESTION: Thanks for your reply.
We cannot easily run an extension cord from the house to the pond.
Will a PVC (Plastic) open pipe from the pond to the outside air, serve to keep the water oxygenated enough?
Will the exposure to cooler outside temps (compared to the water temp)decrease the pond temperature and create thicker ice? I'm concerned the goldfish won't have enough room to swim around.


Answer
Hello again, Noelle!
As I have stated before..the fact that ponds are exposed to so many outside influences makes it impossible to predict how things will go if standard practices are not/cannot be followed.
The water inside the pvc pipe will freeze also, if it gets cold enough. Any drop in outside temperature will reduce the water temperature. And yes, we have seen completely frozen ponds, which would freeze your fish also.