Most ornamental fish are types of goldfish or koi that require almost no care besides daily feeding. Be sure to pick a location that gets a balance of sun and shade, away from deciduous trees; decomposed fallen leaves can not only suffocate fish, they'll also muck up the water. The site should be clear of any rain runoff that can carry fertilizers or yard debris into the pond, where it may harm plants and wildlife. Two to 10 gallons of water is the minimum for a few small goldfish; larger koi require 1,000 gallons or more. Depth matters too: if the fish are going to winter over underneath a layer of ice, the pond should be at least 36 inches deep. You'll need to test the chemical balance of the water periodically to keep it healthy for fish and plant life.