The objective of carrying out a partial water change is to maintain a stable, quality pond environment for your koi. This means that frequent small water changes are preferred over fewer, larger water changes.
Another reason for carrying out a partial water change is to add minerals to a pond that may not have had the benefit of regular clay treatments. If you're from a hard water area, then a regular water change will help to refresh the mineral content of your pond.
The vast majority of us depend on tap water as the source of the pond water in which we keep our koi. This is the same tap water that we use to drink, cook, wash and bathe in, and our water companies claim that it is amongst the safest and highest quality mains water available in the EU (yet the sales of bottled water have never been stronger!). At that same time, we are told that the quality of our tap water is the underlying (and often over-looked) cause of many chronic health problems in our koi.
On the face of it, tap water is crystal clear and free from suspended particulate matter. But because we want our subsequent purifier media to the act long term at the molecular level, we must ensure that any microscopic debris is prevented from impacting on the purifying media later on. A typical first cartridge will trap particles down to 5 microns. These usually take the appearance of at tightly-bound spool of white wool. But take a look at it after only a month or so and it will have a muddy red or brown appearance, largely due to the interception of iron. These pre-filters will have to be replaced periodically. Carbon filters are the next to process the water. The carbon filter removes a host of contaminants such as chlorine, pesticides, herbicides and other organic materials that could not be removed by the latter cartridges.
AS all fishkeepers know, it is necessary to remove chlorine and other impurities from tap water when introducing it to a pond or aquarium. Failure to do this can result in fish health problems and it also adversely affects the growth of benefical bacteria in all filters. The most common method of de-chlorination is to add chemical treatments to break down the chlorine i.e. dechlorinators.