Greetings across the pond!
Looks like you have some koi in there too,
very pretty.

I noticed a pipe that looks like it takes water
to a waterfall. Is there anything back there with it that looks like a filter?
It would have some kind of media in it like brushes, foam pads, funny looking
little balls.
If you find something, you can describe it and someone will help you with the
care of the filter.

Plants help a lot towards keeping a pond healthy.
You can look for a nursery nearby that stocks plants. The tags should say at
what level the plant should be placed in the water. Lilies on the bottom,
marginal plants with 5ish to 1 inches of water over the top, floating plants
like to float.
There are also some grasses and like plants that like to be totally submerged.
All these plants will help filter the fish waste from the water. They will
convert fishy ammonia into plant friendly fertilizer. You can buy test kits to
read your water quality or take a sample to a fish store or nursery. Often
other pond keepers know who has the best service and knowledge.

When we clean our pond we remove the fish to a 150 gallon stock tank, then we
shovel the gunk off the bottom. You need to treat the water from the hose when
you put it back in to neutralize the chlorine. Some townships put in
chloramines which need a different treatment. You can call your water supplier
and see what they add. A fish store or nursery can provide the additives.

Fish tend to make more fish. We usually recommend 1000 gallons (all US
measurements here) to begin with for koi and then 100 gallons per koi after
that. Goldfish need 20 gallons a fish. When your stocking level exceeds those
recommendations you need to give away excess fish. People love to adopt koi, I
managed to thin my pond considerably by notifying the local koi club that I
had koi for adoption. Many people took goldfish too.

Over population and overfeeding in a pond causing much of the problems that
people encounter with ponds.
For feeding the rule of thumb is whatever the fish can eat in five minutes and
then net out any left over food. You'll get a feel for what the fish can
handle. A couple times a day, or morning and evening and you will have happy
fish. They can be very entertaining to feed.

I'm sure others will jump in here with whatever
I've forgotten!

kathy :-)