Most of the inexpensive photoswitches I've seen are for lighting and work opposite - on at dusk and off at dawn.
You can use an Intermatic digital outdoor timer in your application. Check www.intermatic.com. They make indoor timers that automatically adjust for changing sunrise/sunset... I have one, it works well and would probably work for you if the on/off switch for the fountain is inside the house. If the device has to live outside, call intermatic to see if they have a recommendation. Maybe they have an outdoor timer that does this also. Or, you could just go with one of their straight on-off outdoor timers that would turn the fountain on and off at fixed times.