When it is time to change the water in any environment that you have fish in it is not a great idea to change all of it. Fish do not like to have any changes that are sudden in their environment. Any change such as temperature, pH, or chemistry to the water. If you have a good filtration system it should have a biological filter and also a mechanical filter. This is just like water found in the wild. The filter that removes the fish waste that is dissolved is the biological filter. The filter turns the waste into nitrite and then it tuns the nitrite 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 ponds found in nature. Water changes should be done often however they only need to be changed about ten percent every two weeks. When you do your water changes you should also check for nitrate levels this will prevent any discoloration in your koi pond. You can test for nitrate using a test kit that will let you measure the nitrate level, pH, nitrite, and oxygen level in your koi pond. Similar to ponds 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. Chlorine will come out of the water if it sits for a few hours also.