Apparently years and years ago there was a pond there. I dug out around 2 feet of muck to get to the bottom of the original pond that was there.
I have shubunkins, they are goldfish that look like Koi but don't get as big and are much easier to take care of. It is "mostly" self sustaining. I have 2 filters and 2 pumps that I have to clean out, during the summer it's every 3-4 weeks, fall and winter it's maybe every 2 months. The cleaning takes about 1/2 hour. I don't treat the water or change it, rain adds usually and I'll occasionally throw a hose in the waterfall area and top off the pond if it gets low but that's pretty rare. The fish are fine foraging for algae and don't need to be fed but they come running like kids chasing an ice cream truck when I do feed them so I do throw them some food in the morning because it makes me smile. Lol
The biggest pain is during the fall. I have a huge tree that sheds come fall so last fall I finally got a net for it. Here's a few pictures of the year before.

