"John" wrote ...
My pond goldfish are dying. Possible causes?

? The heat
? Filling up pond with tap water after evaporation due to heat
? Clearing out plants clogging the pond
? Disturbing sediment, including submerged rotting leaves.
? Scooping off duckweed with metal sieve
? Cherries falling into pond from nearby tree

The water seems not to have properly settled since the plant thinning,
and to be a bit murky

As others have said, you chose the wrong time to have a go at your pond. In
hot weather there is a reduced dissolved oxygen level in pond water, a
thunder storm would be even worse IME.

Topping up the pond with tap water isn't a problem other than it contains
nutrients for the rapid growth of algae thus reducing the oxygen level even
more at night when your water plants do not produce oxygen but carbon
dioxide. I have never used any chemicals to counteract tap water even when
topping up with a 1,000 gals or so. However, your Water Co does flush the
water mains every so often with an insecticide ( not well publicised!) to
kill of the little shrimps that live in the mains, if you top up during a
flush you will lose all your fish, it's why I phone them beforehand to
check.

Clearing out the plants will also reduce oxygen levels during the day but
may help at night if the pond was choked with them (see above).

Stirring up the sediment releases the gasses of decomposition into the
water, reducing oxygen levels.

Scooping off duckweed will increase light levels in the water helping growth
of algae but will also help water plants to produce oxygen during the day.

Anything falling into the pond will produce more gasses of decomposition.

I bet your pond water Nitrite levels are very high if you get a test kit.

A temporary fix for now would be some way of increasing oxygen levels in the
water, especially at night, so as others have said, an air pump and airstone
or pond pump and fountain. (I have my venturi working 24/7 in this hot
sultry weather).
A better more permanent fix would be to thoroughly clean out your pond of
all muck, replant the plants in their pots, install a filter system with
pump running 24/7 and the outlet splashing back into the pond, or if it's a
pumped outlet, use a venturi.

--
Regards
Bob Hobden
17mls W. of London.UK