If you own a pond with fish inside, you'll notice that their behavior tends to change a lot when the cold winter months begin to arrive. When the water temperature cools down, the fish have to adapt to survive to the different conditions. Their bodily temperatures also drop along with the water. Their breathing rate lowers and they tend to move less as they don't need to use up as much energy. Basically, when the temperature of the water is lower, the fish become less active. If you install a heater that heats up the water of your pond, the fish will become a little more active, but the temperatures will still be relatively low, so you shouldn't expect them to be swimming around as quickly as they would be in the summer months.