When the water is colder, their metabolism slows right down. Their heart rate and breathing slows right down too. So they hardly breath or move much. On the other extreme, a goldfish kept in warm water increases the metabolism, it will eat more, grow faster and have more energy, although it will also die sooner than it would in cooler waters.
Well ice is less dense that water,so it will float to the surface cause it to have a colder temperature,that means the temperature at the bottom will be warmer allowing organisms (fishes) to survive in frozen water.
When the water freezes the ice is less dense and the more deep you go into the water the warmer the temperature is so that also allows fish to live.