Fish need access to oxygen in order to survive, make sure there's a hole in the ice that allows the air to make contact with the water. You'll want to be careful while doing this. Whatever you do, don't bang on or hit the ice in order to create a hole, the vibrations and sound waves from this can be fatal to fish. Instead, you can use small amounts of boiling water to melt the ice without hitting or smashing it. A de-icer can be used in order to prevent the water from completely freezing over, but this can use a lot of energy. Utilize it only when you need to, for example if you're going to be gone for a while. Aerators can also be used to inject oxygen into the water for fish and plants to utilize. This will require more equipment than simply melting the ice, but can be a major benefit for maintaining water quality and oxygenating the water during the winter.
It may be tempting, but avoid the urge to completely remove the ice covering your pond. Not only does the ice insulate the water below it from sub-freezing temperatures, but it provides important cover from predators when the fish are moving slowly and aren't able to respond quickly to threats.
