We've had three over forty years.
The first was little more than a "damp window box," on the crazy terrazzo patio I built in 1976. I built a brick quadrant to hold a pre-formed plastic liner.
In 1985 I decided to build a proper 18" deep goldfish pond, with a rockery to one side, plus waterfall, remove all the terrazzo and replace it with crazy York stone together with a new path and a smaller patio behind the shed.
Just a year later I got the "koi bug" and scrapped the goldfish pond, and dug it out to 5ft.
It has a purgeable pump sump, the "trickle change" drain pipe runs from the overflow in the side of the pool, under the pool collar to a house drain.
I built filters out of 4 X 40 gallon water tanks stacked in pairs in a room I built at the back of the garage which also contains a 300gall quarantine tank. The pool also has a UV sterilizer and air pump.
Job done. I also built our tea-house, lanterns and the little pagoda in the following year.
My wife insisted on a rail alongside the pool, she was concerned that she might fall in when walking past it, so I incorporated it into a pergola. A week after I built it I came home from work to find her on top of the rockery pruning stuff on the fence!
Little has changed apart from the plants that grew and those we've added, in the ensuing years. I also replaced the pergola with a more substantial one.
And the "dwarf" conifer on the rockery is now about 30ft tall but we like it.
Beautiful! Looks like such a beautiful and relaxing area. You have done a great job with it. 
