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