We installed a pond when we moved to CET Towers in 1996. It's 3m x 1.5m x 60cm deep, which was a very big hole, and took some to dig, although we were lucky to be a back corner plot where they'd heaped up all the top soil. Around the edge we put a layer of stone chippings, so most of the soil drainage goes into that rather than the pond. To oxygenate it we have a pump which feeds a waterfall, with about a 45cm drop (that's where much of the soil went, to be the base for a rockery). It has a custom stainless steel grid over the top, which cost a fortune, but replaces the plastic mesh which was there to keep herons, cats, hedgehogs, and children out.
Every year I pull out 90% of the floating oxygenating weed, and cut back the water mint all the way to its underwater pot. There are also irises and a huge marsh marigold which we planted in 1997 and still survive. There are goldfish in there, which we don't feed and are a self-sustaining population. There are always frogs. I've seen toads in the garden and I sometimes find newts when I'm cleaning out the pond, including a great crested newt one year.
The key things are: 1) having a pond big enough, so that it can be self-sustaining, 2) the water flow, to keep it oxygenated, which also seems to help avoid mosquitos, 3) the annual clean, so that it doesn't revert to bog.
I would expect its probably only about 45cm deep now, with the rest being a steady accumulation of mud, but that's not a worry, it just encourages more stuff to live in it.
