The use of chemicals to sterilize ponds should be avoided
The best ponds are biologically balanced. This requires oxygenating plants: Ceratophyllum demersum (Hornwort), Elodea canadenis (Anacharis), Hottonia palustris
(Water Vioilet), Potomageton crispus (Curly Pondweed), Callitriche verna (Starwort), Fontinalis antipyretica (Willow Moss), Myriophyllum spicatum (Milfoil)
It is best to fill the pond with tapwater (to avoid the water being too acid, too alkali or too rich in nutrients (fish food can also make water too rich in nutrients)
Do not let water run from peat moss into the pond - it will make the water too acid
Be sparing with the use of pumps: they kill microflora
Consider placing an ornamental heron by the pool - to deter the gluttony of real herons
Do not put real goldfish in ponds: they churn the mud and eat the water plants