I keep mine in a tropical tank and it thrives. He is a ruthless and fish, but also extremely interactive. Mine gets a 10g all to himself. They do well in water between 70-80, but can do ok in water down(and a little bit below 60 degrees. But when it gets that low they slow down and are more prone to illness.
I would never keep a paradise fish in a unheated pond, unless you live in a warm tropical climate were the water will naturally stay around 70 degrees.
Elrohir the plakat
Galah cockatoo & African Grey