Koi Survival. Dissolved Oxygen in a Koi Pond

Page Summary:  Fish and biological life in a pond system both natural and artificial need oxygen to support not only the fish themselves but also the myriad of other creatures and  aerobic bacteria responsible for pond life diversity and for  purifying pond water. In nature the only way oxygen can enter the water body is through rain or the interface between the surface of the lake and the air itself. The action of wind in creating waves provides a very efficient way of adding oxygen to water in a lake or pond. The efficiency of aeration is made possible by the large surface area between water and air compared to the contact area on a still day. Conversely on a still hot day the air and water surface interface is minimised and this is less than optimum for oxygen transfer from air to water. Rivers are naturally better oxygenated by virtue of riffles and waterfalls. To replicate this action in a pond air pumps and air stones are used or quite simply a waterfall or cascading fountain. The trick to getting more oxygen into the water is to increase the surface area of water in contact with the air... ie create turbulence.

Air, Oxygen & Aeration

Something you probably won’t come across at most koi fish stores is a kit for testing the dissolved oxygen in water for your pond.

Often, the importance of having a sufficient dissolved oxygen levels in your goldfish pond is understated. One of the biggest problems facing koi and goldfish in our ponds is low levels of oxygen in water.

The most common reason we experience too low a level of oxygen in water is because  we simply don’t have sufficient pond aeration in place.

The use of algaecide to kill off the algae can have the serious the side effect that the level of oxygen in your water will drop significantly, so you need to be very careful in treating algae.

The easiest and most cost effective way to add oxygen to pond water is to use the pump power to create a waterfall.

As water cascades down the waterfall the large interface between water and air allows for maximum oxygen transfer from air to water.

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Treating Low Levels of Oxygen in Water

Pond aeration is crucial because, unfortunately, unlike with nitrate of metal poisoning, there is no way to treat low dissolved oxygen levels when you notice your koi are stressed and starved for oxygen. Poor pond aeration that results in low dissolved oxygen levels can kill your koi or goldfish in less than an hour, and if you don’t catch it almost straight away, your fish will almost certainly die. You can spot goldfish and koi that are struggling with low levels of oxygen in water because you will see them gasping for air at the water’s surface. Within just 10 minutes, your koi will not be able to control their swim bladder, thus struggling to remain upright. From here on in, critical damage to the koi’s internal organs wil occur, resulting in death within 45 minutes.

Good Pond Aeration for Maintaining Dissolved Oxygen Levels

It can be a tricky challenge to preserve the level of oxygen in water. Typical koi and goldfish ponds contain a large volume of water, with several pockets of deep water. Most people struggle with the level of dissolved oxygen when trying to setup a new pond aeration system, or if they are trying to remove algae from an existing pond aeration system. However, it is possible to maintain high levels of dissolved oxygen in water by applying several inexpensive solutions.

The most important aspect of good pond aeration is circulation. This needs to be as frequent and as high as possible. Many people seem to place a lot of importance on using air stones and air bubbles in their pond aeration systems, but while these help, they will not keep the level of oxygen in water sufficiently high enough by themselves. What matters most of all is breaking the surface of the water, as this is what allows oxygen exchange to take place. The level of oxygen in water will constantly be replenished by movement across the water’s surface, and so a good pump, assisted by oxygen for biofilter or even a fountain if possible, will go a long way.


One more pond aeration trick you can do to ensure good levels of oxygen in water is to keep lots of plants in your koi or goldfish pond. Plants will fight the algae cells for what carbon dioxide there is in your pond, all the time releasing oxygen as a bi-product. It’s recommended to keep at least 1/3 of your pond covered with plants for good pond aeration. You can use either potted pond plants or you can deploy floating pond plants for pond aeration, such as water cabbage or water hyacinth. Virtually all successful koi and goldfish ponds contain numerous plants to help with pond aeration.

Replacing Water to Maintain Dissolved Oxygen Levels

It’s very important for your koi’s and goldfish’s health to change the water regularly. Koi and goldfish are quite messy fish, and so for a pond aeration to work in a pond that is well stocked, it’s necessary to change at least 25% of the water every week. Water changes are vital because fresh water is always oxygenated with high levels of dissolved oxygen, and it will help considerably in keeping your pond aeration stable. Out of all the methods of pond aeration we have mentioned, changing the water is the most important one.

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