Fantails can be kept in outdoor ponds-- as long as the conditions are right. They are slow moving and more prone to predation, but as long as you live in a temperate area and/or take them in in cold weather, there is no reason not to keep them outside if you wish. They are not as hearty in the winter as regular pond goldies (comets) and prefer warmer temps-- that, as far as I know, is the only contraindication for fantails in ponds. They get beautiful and palm sized when the conditions are right.
Nice goldies, BTW!!