Actually, if a pond is totally frozen solid, the fish won't survive. There is only so much dissolved oxygen, and once that is depleted, it's goodbye fish. The length of time this takes will be variable, but isn't long. Photosynthesis is unlikely to take place much either due to a) an absence of a great deal of light and b) an absence of carbon dioxide to photosynthesize. This why fish pond owners are recommended to leave a small hole in the ice. You can get rubber rings and the like to ensure an opening is maintained so your fish don't suffocate.
Andy