Bart, That discussion is being made over and over again on just about every Koi, Goldfish, and tilipia pond forum out there. I had a pretty long page typed out and was doing some research to find good correct answers for anyone that might read this. problem is the more you read the more you learn and the more you have to decide what works for you. In Japan, The best breeders will tell you that they are not fish keepers, they are the water keepers and the fish live in the water they keep. I've been keeping fish for over 30 years. I've been taught over the years that the best way to reduce nitrates and phosphates was to remove some of the old water and replace it with new. Some fish and and systems require large changes, In most cases though that is not the case unless there is something going wrong in the system. smaller 10% changes are better for the actual change, reduces the nitrates, phosphates and dissolved organic compounds. DOC's
While searching I found this article, it explains the reasons for water changes pretty well. But if you do more research you will also find that there are breeders that do not do regular water changes, they do however in most cases have filters that equally as large as the pond itself. This is why they can get away without them as they have designed systems that remove these products from their system and again, they are the water keepers, The fish just live in the water the take care of.
Also keep in mind when reading the article that they refer to Koi, Koi are carp as well as Goldfish, Just larger, same principles apply for both just more critical with Koi as they grow three, four five, maybe even times as large and put out three, four, five or more times as much waste and DOC into the water as the equal # of goldfish would produce.
Koi ponds are a closed system with far more of a bio load then any natural system. Without the replacement of water on a regular basis the system will quickly become overloaded with organics. The koi will suffer in both their development and in their health.
One quick note before I get too deep into this subject. Adding water to a koi pond to replace water that evaporated does not qualify as a water change. Yes you are adding fresh water into the system, but you are not removing any dissolved organic compounds (DOCs) from the system. So as you go along adding water, the DOCs continue to increase and the environment worsens. A proper koi pond water change requires that you remove a certain percent of the water and replace it with fresh water.
Water changes are a crucial part of koi husbandry. By changing water you are reducing the organic load in the pond and increasing the mineral content. As organics build up and minerals get consumed, the water slowly becomes unhealthy for the koi. Eventually the deteriorating water conditions will claim their lives if left unchecked. Also, koi produce a hormone into the water and when it reaches a certain density it will trigger a biological change in which the koi will stop growing and stop producing eggs. This is mother natures way of preventing over population in a lake if for some reason the inlets stop flowing. So it is very common for koi that are kept in crowded small ponds that don't get a lot of water changes to simply never grow or only grow a few inches. Increasing the amount and frequency of the water changes will remove this hormone as well as the organics in the water providing a better growing environment for your koi.
Without water changes you will start to see very high levels of Nitrates and foam may appear on the surface of the koi pond. These are two clear indicators that it is time for a water change. In fact if foam appears you are well overdue for a water change and a larger amount then normal should be removed. In general a small 10-15% water change per week is more then enough. For many, simply backwashing your filters will remove enough water for an effective water change. If you have a heavily stocked koi pond and/or an undersized filter, then you may need to do as much as 20-30% water changes several times a week. Every koi pond is different and you will need to establish a regular routine that works for your pond.
Time of year will also effect the frequency and the amount of water you change. During the winter, the koi are not eating or producing much waste. So depending on your climate, you may need to do a water change as little as once a month or not at all during the colder months. But as things warm up we need to start a more frequent routine. During the peak summer season when the fish are eating heavily and the organic load is at its peak, this is when you will need to do the most frequent water changes and filter backwashes. During this time the once a week water change is a bare minimum, not a maximum.
There are those who live in drought areas and simply cannot in good conscious throw away that much water. In situations like this there are solutions. The easiest is to simply reuse the water you are draining away. Plants, grass and fruit trees will love the nitrate rich water. On the day you plan to backwash, turn off the sprinklers and use the pond water. One more thing to keep in mind during a water change is temperatures. Major water changes can cause significant shifts in the temperature of the koi pond. Major swings in temp can at the very least be stressful to the koi, and at times, lethal. In general koi do much better going from cold to warm, but not so well going from warm to cold. So during the summer you need to be careful about major water changes. If the koi pond is say 80 degrees one hot August day and you do a 50% water change and the tap is a chilly 65 degrees, well you could easily shift the water temps 7-8 degrees in a matter of minutes. The koi will not be happy and may rush to the bottom of the pond and stop swimming. It is best during the hot months to do smaller water changes more frequently. But if you are in a bind and you must do a major water change for some reason, then try to add the water slowly. If you can spread the water addition over a period of an hour or two, then the temperature shock on the koi will be greatly reduced.
A few tips to keep in mind when doing water changes:
1. Always use dechlorinator. With major water changes a dehlorinator and ammonia binder is best.
2. Don't forget the water is running!!!!! The number one reason for total loss of all koi in a pond is forgetting to turn the water off. The chlorinated water will burn the koi's gills and they will die within a couple hours.
3. Always add a bit extra dechlorinator just in case
4. If you use a carbon prefilter, you won't need dechlor, but monitor the carbon and replace when needed
5. Sometimes the city will flush the lines with elevated levels of chlorine and chloramines to clean out the pipes. I always add extra dechlor, but you if you are doing a major water change, testing for chlorine levels before you add the water is a good idea.
Regardless of how you decide to do your water changes, they must be done if you want your koi to live long healthy lives and grow to their full potential. In a well designed koi pond a water change is a quick and simple procedure and usually combined with the weekly backwash routine. Just take care and be aware of your actions. Water changes can become routine and humans are a forgetful species.
