The photo above is that of a Koi (Japanese carp). This one is a Goromo - one of the many different varieties of Koi . One of my first and certainly not my last.

Monday, April 6, 2009

Must you change the water in your Koi pond? How often do you do it? How much to change each time?

These are questions often asked by relatives and friends when they see my Koi pond. Whether it is a pond or an aquarium, regular water change is necessary. Actually, partial water changes and it must not be more than 50% of the water at a time. This is because fish (Koi included) does not like sudden changes to their living environment. Furthermore, I have to refill my pond with tap water from the utility company. The tap water contains chlorine to kill germs and it is also a fish killer. I have managed to do up to 40% water change without using dechlorinator which removes the chlorine from the tap water. Any more could be risky for my Koi, a lesson I painfully learnt.

Firstly, why are water changes necessary? Doesn’t the filter remove fish waste and other debris like uneaten fish food, dead insects and plant matters? Yes, a good filter should be able to trap solid debris through mechanical filtration and allows you to remove them easily during regular filter maintenance. The filtration system should remove dissolved fish waste like ammonia by converting them to nitrite, and then from nitrite to nitrate. This is done through bacteria action in the biological filter. Ammonia and nitrite are harmful to fish and must be removed as soon as possible. Hence the need for a well designed filtration system. Although nitrate is relatively harmless to fish, excessive nitrate when allowed to build up in the pond can still harm the fish and also cause problem of excessive algae growth. Water plants remove nitrates. Although I have included a water garden in my pond design, I doubt the plants can remove the nitrates at a sufficient rate compared to the amount of food consumed by the Koi each day (= waste produced). Therefore, even with an excellent filtration system, partial water changes is necessary to remove the nitrates. I notice my Koi becoming more active and lively after a partial water change.

Next, how often do you perform the water change? If you have a large pond, intervals between water changes can be longer. I do it at least once a week although my pond can withstand a longer period without any water change. This is because my filter design (with the bottom drains in each filter chamber leading to a sludge chamber holding a drain pump) makes this an easy task. While making the partial water change, I would flush the bottom drain of each chamber in my filtration system. It takes only a few minutes and about half a cubic meter of water is removed each time.

To reduce the amount of tap water needed for the refill, I would try to do my partial water changes when rain is imminent. I do get a lot of rainfall in Singapore during the wet season.

On a quarterly basis, I would do my filter maintenance and flush one of the filter chambers and at the same time do a larger volume of water change.

But not more than 50% at a time!


  1. Interesting article. I hadn't realised how much work was involved in maintaing a pond! I thought the filter system did most of the work

  2. Depends on the capacity of your filter system, if it is big enough I(>1/3 the size of pond), regular changing of water is unnecessary and may even harm your kois if you forget to turn off the tap (always use dechlorinator solution).

    One of the purposes of changing water is to replenish the minerals in the water (rain water is mineral free but not pollutant free) as the kois require minerals to remain healthy. This can be overcame easily by adding huminic or refresh powders, which simulate the chemical composition of a mud pond, regularly.

    I do not change water regularly. I only top up during the dry season. I let my filter system do the work. The filters are washed about half yearly (used to be yearly). Also I rely entirely on coral chips and cockles shells. Natural carbonates stablise the water PH. ;-)

  3. As for the Nitrates...if the algae growth in your pond is in control, you should not have nitrates problem.

    Algae is good for Kois...especially the Hi in the kois. You will be surprised the amount of algae your kois can consume...


  4. Having said that, let me qualify that this only applies if you have sufficient sunlight and nitrate absorbing plants in your pond. For those whose ponds are indoor, do follow Jlee's water changing regime. ;-)

  5. I do my water change once a week although I don't really have. Somehow, the filter design makes it so easy and convenient that it has not a chore.

    It is more to flush the bottom my settlement chamber, than to remove the nitrates. I notice that I flush out about half cubic meter of blackish water each time. So I am killing two birds with one stone - removing nitrates and also debris at the bottom that may breed patogens that affect the koi.

  6. AM I right in saying every water change should be about 15% - 20%?

    1. Amount of water change each time must not be too much as to cause sudden change in water condition. 15% to 20% would be a good guide. I have done as much as 40 to 50% but not more than that as the risk to the fish goes up as the percentage water change goes higher. Also, if you are using tap water to refill your pond/tank, the water needs to be de-chlorinated. Changing less than 20% of water each time also dilutes any chlorine that are present or remaining in the water.

    2. can you do a water change of 10-15% directly from the filter

    3. Yes. Changing water is a good opportunity to also flush the dirt collected at the bottom of filter. My filter has bottom drains with pipes that lead to a drainage chamber with a drain pump to expel the water out of the filter.


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