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!