Your 1st objective is to have sufficient dissolved oxygen in the water so
that the fish and beneficial bacteria could live well. Which means you
need to aerate and circulate the water. The rest of the listed items are
there to help clear the water. The list would be longer, pending on the
type of the pond.

You need to google so that you could decide which type of pond you want,
then people here could help you to set it up.

If i had a pond that size and location, I would get a pump to
aerate/circulate the water, put in a lot on plants, then throw in a few
Koi. If you like to spend $100k like someone I met, you need much more.

Jeff wrote:

We have a large spring fed pond on our property - 45x55, that we are
trying to make into a koi pond. It has a couple of trees around it but
mostly sunny area. We are getting conflicting advise, from the need
for filters, liners, caves, ledges, plants vs no plants, etc to being
told just circulate the water and throw in the fish. Where can we get
good advise - most of the books seem to be for much smaller ponds. We
are in Upper Makefield PA, 35 mi north of Philly Thanks
- Jeff