A hole in the ice is essential so that your fish can breathe and gasses have a chance to escape, but breaking the ice by hitting it is never the answer. Fish are incredibly sensitive to the vibrations formed by hitting ice atop the water, and can even be killed from the sound waves. Instead of smashing or cracking the ice, use boiling water or a deicer to melt the ice. And of course, always have a proper aerator working at all times in order to keep that hole open and provide your fish with necessary oxygen.
During the winter, check your pond at least once daily to be sure everything is in order. If you've got a think layer of ice forming over your pond and the temperatures are forecast to continue falling, go ahead and turn on your deicer for a few hours per day, especially in the coldest hours of very early morning. A timer would be a great tool for this. Deicers use a lot of energy, so it's not ideal to keep them running nonstop. If you're going to be away for a few days and the temps will remain below freezing, it may be worthwhile to keep that deicer on. The good news is that fish won't die immediately if the ice hole begins to freeze over - it's the buildup of toxic gases that's the problem, and you have time to head out with a pot of hot water to melt the ice and set up a small aerator. Never break ice in a fish pond by force!
