I thought about getting an aerator, but not at the risk of harming the fish. How did the fish die by using an aerator?
An aerator won't harm your fish - we have aerators that are set to turn on at night during the warmer months to increase oxygenation in the pond. Our fish actually love when the bubblers come on - they take turns swimming through the streams of bubbles like a bunch of kids playing in the sprinkler in the yard.
I think what you read is that someone was relying on an aerator to keep a hole in the ice during the winter months. In some climates that may be enough, but if you live in an area where you get a heavy layer of ice on your pond (like MitchM) and aerator wouldn't begin to do the job. The pond would ice over completely and the fish would be at risk of dying due to the build up of gases under the ice.
The year we shut our waterfall off for the winter, we used a pump set about a foot under the surface with the outlet directed upward to create a fountain effect to keep a hole open. Two things happened - a large dome of ice formed over the water that was being sprayed into the air and the pond felt constantly agitated to me. Not at all the desired effect. The fish were fine come spring, but we did worry.
We have also used a deicer set over an aerator that was just under the surface of the pond - that worked better, and seemed calmer to me. (I don't know if the fish cared, but I did!) The past few years we've kept our waterfall running all winter - the best solution (for our pond) of any we've tried.
So don't fear the aerator! Just know your purpose for using it and make sure you are choosing the right tool for your intended purpose.