If you live where the winter temperatures at night will not drop below about 10-20 degrees below freezing at night, then you have a couple of options to help your plants and fish survive and keep the water from freezing:
Fountains create enough movement in the water to keep it from freezing, just make sure not to exceed the manufacturers minimum operating temperature, you don’t want your fountain head or pump to break. I wouldn’t try operating your fountain below about 20 degrees or so.
A better option (especially for plants and fish) would be to submerge an aquarium heater on the top edge of your container, just like you would use on an aquarium tank. A bird-bath type heater would also work. A submersible heater with a thermostat will allow you to control the temperature of the water automatically, this way you don’t have to monitor it. Be mindful of the weather and not leave heaters made for indoor use outside in rain or snow. Using a G.F.C.I. or outdoor protected outlet is recommended.
Of course you can take your plants and fish indoors for the winter months. Using a thermostat/heater will extend your gardening season even if you live in the extreme north where over-wintering aquatic plants and fish outdoors in a container is impossible. I hope these ideas help you enjoy your water gardens and fountains a month or two earlier in the Spring and later into the fall without having to worry about early and late season freezes.