Do you know how deep your pond is? Too shallow and the fish won't survive. I am using an air pump to keep a hole open to allow for the exchange of gasses. A heater is way more expensive (cost of electricity) compared to an air pump. The heater I looked at is around 1400 watts compare to 40 watts for the air pump. Hope this help.

I think it's at least 5-6 feet deep or more toward the middle, despite our losing a lot of water this summer.

It's so far away from the house that a heater isn't a feasible option and we can't afford an aerator right now. Is it true that just brushing the snow off the ice facilitates gas exchange?

We have two perpetual turtle residents, at least 2 goldfish and countless minnows who have successfully overwintered for the last couple of years...I'm just worried about this year's crop of goldfish being too large. We usually have several herons that take care of this problem, LOL, but maybe their aim has been bad this summer. Too bad I can't sic my cat underwater!

I don't think just brushing off the snow will be enough. You need to maintain a hole in the ice for oxygen.

That's very deep for things to still be dying so much over the winter. Maybe it was just nature weeding out the weaker ones?

Do you know how many fish you have total? A large pond like that should be able to successfully maintain a lot of goldfish.

I would just make sure there isn't too much debris on the bottom (releases ammonia and nitrites which build up and cause problems) before freeze up, and try to keep a couple of holes open as much as possible. A cheap way to keep holes in the ice is to put pots of boiling water on the ice until it melts through, but that can be labour intensive in very cold weather.

I am not sure if you want to try a windmill aerator. You can place it close to the pond. Just a suggestion. I have never used it but some farmers do. Not sure how expensive it is. Search the web and I am sure you will find many options.

Is this a liner pond or a dirt lined pond? If liner....what kind of filtration is in there? How much decaying debris is in the bottom? As debris decays in an anaerobic environment, hydrogen sulphide gasses are formed (along with ammonia). Hydrogen sulphide is very toxic to all forms of aquatic life, including frogs. In addition to making sure a hole is kept open for the winter, you really need to get the decaying material out of that pond and make sure your fish load is at a level the pond can handle. If you try to overwinter too many, you may lose them all. And as fish do die, conditions become even worse with more ammonia levels.

My husband does this because we have swans,they need an area open,We have a generator in the barn,he has tubing going from generator to the pond.on the end of tube he took a piece of pipe ,drilled holes in the length of pipe.Put this end in pond.Turn on generator that is on a timer.It keeps a big section open in ice.
We have never had a winter fish kill but we had a summer fish kill hundreds,cleaned the pond out nicely,heck of a way to do it though,especially 3 days before your sons wedding to be held at the pond on the gazebo,we ended up burying the fish.