What I have considered is suggesting to them that they turn it off at
night.
I don't mind it so much during the day because my area is not particularly
quiet so it sort of blends in with the general background noise more.

The problem is that I anticipate they will tell me "where to go" if I
request
this as that does tend to be their style rather...

So if they refuse to turn if off at night is there anything that can be
done
about it?

It may actually not be possible to turn it off for very good reason.
Depending on how their pond is built & plumbed it may well be that the
waterfall is actually the outflow from the pond's filter - this is quite a
common thing to do. If this is the case then the pump has to remain on 24
hours a day or the filter will die & turn nasty.

Another option would be to re-plumb the filter to return quietly directly
into the pond at night but this would require excavation, pipework &
general expense - you'd also be reliant on someone remembering to actually
switch the thing over every night.

If this is not the case & it is just a decorative waterfall then perhaps a
polite chat & an offer to pay for a timer plug might solve the problem!

Worst case, look on the bright side - at least its not Wind Chimes!

rgds

I

For those non aquarists (& anyone else who is interested!) pond (and fish
tank) filters work something like this....

Although there is some Mechanical filtration taking place in the filter this
is not actually what's important - what's important is the biological
filtration.

Waste products in the pond give off Ammonia - this is toxic to fish. By
pumping the water through the filter a set of Bacteria build up in the
filter that eat the ammonia & turn it into Nitrite (also toxic to fish!). A
second set of bacteria also grow in the filter that eat the Nitrite & turn
it into Nitrate which is not toxic to fish (except in huge quantities).
Plants in the pond absorb the Nitrate as a fertilizer. The bacteria that do
all the good work need a constant supply of oxygen rich water, therefore if
you turn the filter off they die back - this can happen in a matter of
hours.

Worse still is that if the filter is left off for a prolonged period of time
it will go bad ("Anaerobic"). When this happens it starts producing Sulphur
Dioxide (rotten egg smell) in some quantity - this is highly toxic and will
kill anything living in the pond if it gets in there.....

I.