On Tue, 01 Oct 2013 12:25:22 +0100, Stephen Wolstenholme wrote:

20'x20'x4' But it's got a lot of fish in it and they've
obviously
been breeding, which aiui is supposed to show that they're happy
with
their conditions. We have never had this problem before so the
failure
of the pump has to be the cause, I would be pretty certain.

Probably but don't rule out old age or disease. Are the trees
dropping their leaves down there? If they have gotten into the pond
and starting to decompose it will lower the oxygen levels.

If you want a pump of X litres per hour get two of X/2 litres per
hour. They can be plumbed together with plastic piping. Don't rely on
one pump.

Good, obvious when you think about it, idea especially with a large
pond and lots of fish.

--
Cheers
Dave.