You wouldn’t think that a fish’s life was overly stressful. However fish can get stressed when their environment is not adequate or not constant. This can even lead to fish deaths.
If a pond does not have sufficient oxygen and all pond life is competing for oxygen this can cause stress for your fish. If your fish are at the surface gulping for air then your pond is highly likely to be insufficiently aerated. An air pump is placed on the ground outside the pond. It can be hidden to blend into the landscape around your pond and then introduces air into your pond. You will want to ensure that it is insulated so is not introducing too cold air into the pond which can also lead to stressed fish. The oxygen that is taken up by fish, plants, bacteria etc is then replenished and all pond inhabitants remain unstressed.
The circulation of water created by your air pump will also enable you to keep a hole in the ice over the winter that can form on your pond. This will let undesirable gases escape from your pond which can also keep your fish happy.
If you do want your fish to thrive in your pond consider getting an air pump facilitate this.