When we want to add water to our pond, we just use the faucet with a very low trickle. Chlorine evaporates pretty quickly. If we're in a rush (our pond has leaked a couple of times), we add Pondcare Chlorine and Heavy Metal Neutralizer, which has apparently worked quite well as our five large koi are still alive and kicking.

There is no reason to routinely change your water, however. You'll be messing up your biofilter.

Rainwater will probably work fine (I'm assuming your pond is constantly circulating and so oxygenating). You would probably rest easier if you picked up ph, nitrite and ammonia test kits and checked the water from time to time. Fun little chemistry sets.

This is where the plants and algae come into the equation, because nitrates are excellent plant food. (Algae are a basic form of plant life.) The plants remove the nitrates from the water to combine with carbon dioxide and sunlight to produce more plant.

Fish such as koi graze constantly on the algae, and will eat some of the dead leaves that fall into the water, so the circle at this point is complete.

for more info. please visit http://forum.californiawaterscapes.com

Here is a link that might be useful: pond facts