During the process of buying our house, one of the disclosures was that the house has a huge inground swimming pool, but some aspect of the plumbing was broken, and nobody knew how much it might cost to repair. So, as-is.
Nobody in our house swims, and the costs of repairing and running the pool would be excessive. At first I thought about having it drained and either removed or converted to some kind of outbuilding space. Then it occurred to me, I shouldn't just dispose of a water-bearing structure! The thing is engineered to safely contain thousands of gallons! We kept it. The pool hadn't been used in several years prior to the sale of the house, which was perfect because the chlorination had long since evaporated. Thus a readymade freshwater pond.
I started adding native plants and sheltering areas to the shallow end. Frogs were already in residence for mating season but now several species live there full time. Tons of insects: backswimmers, water skimmers, caddisfly, mayfly, damselfly and dragonfly larvae, bladder snails, giant water beetles. Ducks and herons visit. Purple martins and flycatchers hang out at dusk.
People often ask about mosquito control, and so far it hasn't been an issue. I believe this is due to a combination of the water depth (mosquitos prefer very shallow and still water like puddles), circulation speed, and predation of the mosquito larvae by everyone else. Because there are marginal plants, predatory insects can transition from their aquatic larval stage to adulthood (they need stalks to climb up out of the water).
The pond has become one of the great joys of my life--seeing the seasons change through the lifecycles of the species, watching bees forage on pickerel rush flowers, hearing frogs sing at night.