Fish can survive underneath a layer of ice in a pond, but no fish can survive a pond which is completely frozen from the surface to the bottom. This is because the fish will freeze in turn, which will cause irreparable damage to their internal organs and tissues. Fish also need oxygen from the water in order to breathe, and room to move around to keep healthy.
Fish will also likely suffer if the layer of ice on the top of their pond is completely solid. Oxygen in the water often comes from the air itself, and lack of contact with the outside air will seriously deplete stores of oxygen in the water.
