All Things Koi and H2O
Having dug the hole and placed the container, the next step is adding the H2O. Most tap water has chlorine in it, which will kill fish. Your options are to let the pond sit for 24-48 hours to let the chlorine evaporate, use untreated spring water, or to add chlorine remover (available at tropical fish supply stores).
Natural ponds usually have springs or rivers feeding in and out of them, keeping the water pure. I use a pump and filter intended for aquarium use in my pond.
Most people associate water lilies with ponds, but they can be pricey and as they require a pot with soil, also potentially messy. Inexpensive alternatives include water hyacinths (which produce beautiful purple blooms) and water lettuce; both of which are freefloaters - meaning no pots/soil required. Those "lucky bamboo" plants thrive in an aquatic environment.