Our goldfish have (almost) all died suddenly, and I was wondering if anyone
could help us determine the cause.

They were in a pond of about 3000 litres (3m x 1m x 1m deep). This is the
upper of our two ponds. The lower one is probably about twice the area and
half the volume, and has a pump in it, which produces a fountain, and pumps
water through a filter to the upper pond, which then goes down a waterfall
back to the lower pond. We don't leave the pump on all the time, because
there seems to be some leakage (probably in the waterfall), so the level
drops after a while, but it is normally on for a few hours each weekend,
less during the week, and not at all when we are away. The lower pond has
plenty of plant life, but the upper pond has virtually none. It gets very
little sun because it is heavily shaded by a willow tree and a large pampas
grass. The fish are perhaps not fed as often as they should be (about two or
three times a week), but they seem to have survived on this up till now. We
have noticed over the last few months that whenever the pump is first turned
on there is a nasty smell, but this goes away after a few minutes, and did
not seem to be causing a problem. We were away for a couple of days over
Easter, then when I turned the pump on on Monday morning, I didn't notice
any dead fish, but a few hours later there were 21 dead fish floating on the
surface. Over the next few days another 9 died, though earlier this evening
I did actually see one still alive.

Can anyone suggest what we can do to prevent this happening again, and if
possible to rescue the one (or more) survivor(s)?

Mike.