On Sun, 10 Mar 2013 15:42:48 +0000, Spider wrote:

Since the fish survived one hard winter, I find it hard to believe
they've succumbed to cold this year. There is a bubbling spout to
aerate the water, so that's not the problem.

It might be. Water is funny stuff it's maximum density is at about 4C.
Water at this temperature sinks to the bottom and stays there. The
surface can cool further and freeze but is less dense so stays at the
surface. The bubbles will tend to make the water circulate and thus the
whole body of the of the pond water can get cooled below 4C. The fish may
well object to temps below 4C...

--
Cheers
Dave.