When it comes the time to change the water in any fish enviroment it would not be a good idea to change all of the water at the same time. Any sudden enviroment changes are not a welcome thing by fish at all. Change on temperature, pH, or even chemistry of the water. If you have a good filtration system it should have a biological filter and also a mechanical filter. This is just like ponds found in the wild. The filter that you are using that removes fish waste that is dissolved is the biological filter. The filter will turn the waste into nitrite and then into nitrate. This happen to be done all in the filter system. Nitrate is a chemical that is harmless to the fish but it will let build up produce in your pond that may harm your fish.
Some water plants can be bought to help in the removal of nitrates. But it would take so many plants to remove the amount of nitrate that you need them to. You could try a trickle tower to remove the nitrates in your koi pond however they are not cheap and giving the water a partial change would be more cost efficient. Which is like water found in nature. Water changes should be done often however they only need to be changed about ten percent every two weeks. When it is time to change the water you need to also check for the nitrate level by doing this it will help prevent and discoloration in the koi pond. There are test kits that are available that will let you measure for nitrate levels, pH, nitrite, and also oxygen levels in you pond. Similar to water naturally occurring in the wild.
If you are going to use tap water to replace any water in your koi pond it is a good idea to use additives to remove the chlorine that is added to the water. Also if you have a few hours and a place to store it the chlorine will come out of the water while it sits.


