Koi Fish in a Koi Pond

Which koi keeper doesn’t love a good summer? Sun is shining, your koi are in fine fettle and you get to see them later and later into the evening.

Summer is a great time for koi keepers, but this time of year does come with its own special requirements to help keep an enthusiast and their koi happy and healthy.

Water Parameters

As always, it is the alpha and omega of all koi keeping. The quality of your water affects your koi, their health, their color and their quality of life.

Warmer months bring several challenges in keeping healthy parameters for your koi. For one, they eat more. This means more waste. Which in turn means more maintenance. Then there is the lawn care, the products used to maintain your plants and the extra organic material that can come from those plants.

Checking water parameters regularly is vital to the health of your water and the koi in it.

The frequency of testing the water in a koi pond during the warmer months of the year is a topic for debate. If a koi keeper is attempting to create a water change schedule, testing before the change and then a few days after can be helpful in finding a trend. As can testing when temperature fluctuations are high.

Some keepers might perform a test once a week. Others might do it bi-weekly or even monthly. It all depends on the pond setup and filtration. But any koi keeper that suspects something is out of whack should test the levels immediately.

There are plenty of advertised products that help maintain optimal water parameters, including those that reduce ammonia, reduce phosphates, regulate pH and kH or control algae.

Be sure to research all of the alternatives and the potential downsides to using them. Your pond is a closed system. Anything that goes in will need to come out eventually.

Water changes

When the temperature starts to get higher, so does the necessity for water changes.

water change in a koi pondKoi are poikilotherms regulated by the water temperature they are in—the warmer it is, the more active they become. Being more active means more frequent feedings. So between higher water temperatures, undigested food, and waste increase, the water parameters (especially ammonia levels) can quickly become a problem.

The standard recommendation is at least one 10-20% water change per week. This number can be different depending on the fish load in the pond as well as the nature and size of the filters being used.

Some enthusiasts opt for a flow through (or “trickle”) that constantly removes and adds water.

Of course, in some parts of the country, frequent water changes might not always be possible. There are chemical oxidizers (like Potassium Permanganate) which can reduce the number of water changes. However, they do have downsides, and koi keepers should be extremely careful with the amounts and concentrations used.

As well as removing waste and potentially harmful elements, fresh water will also introduce some trace minerals that are leached out by fish and plants for growth.

*Caution—big water temperature swings can be lethal to your koi. Going from cold to warm is an easier adjustment for koi, so be aware of this when adding large amounts of cooler water to warm water.


This is a biggie in the summer months. Dissolved oxygen is lower at higher temperatures. As water heats, its ability to hold onto oxygen naturally diminishes. There is also the increased demand for oxygen from the organic components of the pond.

At 90ºF, there are only about 7ppm (parts per million) of dissolved oxygen in the water. To put that in perspective, koi start to become stressed at around 4ppm, and, at 3ppm, they begin to die.

This makes power outages particularly dangerous in the summer. With pumps and any aeration apparatus out of commission, the dissolved oxygen will quickly get used up.

And it’s not only your fish that suffer. Beneficial ammonia- and nitrite-fighting bacteria also need oxygen to do their job. Less oxygen makes them less effective in the nitrogen cycle.

Extra aeration can become necessary in warmer months. Outside of a rebuild, air stones and air pumps are the quickest and most effective way to help increase oxygen levels and circulate pond water.


Water temperatures above 90ºF are not comfortable for koi and can result in stressed koi—which opens them up to disease. And don’t forget, koi can get sunburnt, too!

Shade can help keep water temperatures down by as much as 20ºF, which will also help with dissolved oxygen levels and minimizing algae growth. Natural or man-made shade is especially important in parts of the country that reach extreme temperatures in summer months.

There are a couple of ways when you design your pond to help with temperature regulation.

● Deeper is better—the further into the ground, the cooler the pond will be.

● Bigger is better—more gallons are less prone to temperature swings.

● Check the climate—each zone comes with specific build recommendations.

● Allow a breeze—natural breezes help to cool the pond surface. If it is built in a confined area it won’t receive a breeze.

● Location, location—check the number of direct sunlight hours your preferred location gets. Six to ten hours per day is ideal.

Lattice shade over a raised koi pondOther than these pre-build considerations, there are in-pond and out-of-pond shade options available.

In-pond shade:

● Floating aquatic plants (there are a variety of options available, but don’t exceed 60% of the pond’s surface).

● Man-made floating islands (which are pretty, mobile and provide shade).

Out-of-pond shade:

● Vegetation (trees, pond-side plants, hedges, and shrubs)

● Awnings

● Tarps

● Sail cloths

● Pergolas

● Lattices (with creeping plants)

Vegetation Control

If you are using natural shade to keep your pond cool, maintaining it is important. Summer and bloom go together. This means more grass, more leaves and more flowering plant life.

They offer much-needed shade (for your koi and yourself) and look great around your pond, but keep them out of it.

Skimmers are very helpful to this end. It takes the manual out of your labor by automatically removing it from the surface of your pond. Otherwise, they can become more fodder for those elements that negatively affect your water parameters.

Algae Blooms

String algae on a koi pondSpring Algae blooms are not uncommon in koi ponds. But many of the same necessary ingredients for algae blooms are still around in the summer, and in some cases, even more so.

A little algae can be a good thing. Algae thrive off the nitrates in koi waste. They also serve as a convenient snack for hungry koi. But in the summer months, there will be waste aplenty, elevated water temperatures, an increased amount of sunlight—all of which help algae thrive and proliferate.

Not only are blooms unsightly, they are dangerous, too. Algae is an oxygen hog, soaking it up at night, making it difficult for koi to breathe in early hours of the morning.

Keep an eye on those factors that contribute to algae growth and it will reduce the chance of a bloom.

Filter cleaning

Increased waste means that filters will more quickly fill up with material that needs to be removed. This means more frequent filter maintenance to keep them operating at maximum efficiency.

Once a week cleaning and flushing is the minimum to keep your filters running at their peak.


In summer, it is possible to lose as much as ¾” of water per day due to evaporation.

Evaporation cannot be stopped. But it can be slowed. Floating aquatic plants (remember them?) not only help to provide shade and keep water temperatures down, but they will absorb the sun’s energy for their own needs.

Long streams, sprays, fountains and waterfalls will contribute to evaporation, too. So if evaporation is a real issue, reduce the amount of water agitation that any water feature might be causing. Of course, reducing the agitation can affect the water’s ability to diffuse heat, so be aware.

NOTE: Evaporation does NOT constitute part of a water change. The dissolved organic compounds and harmful parts of the nitrogen cycle aren’t removed.
Rain falling on a koi pond
Courtesy: lucksey


Some regions will receive a significant amount of rain and water additions. Some enthusiasts will even use the rain for water changes.

Although not a major cause for concern, rain can prove to be a challenge—especially with regards to maintaining optimal water parameters.

Rainwater and runoff can add unwanted inorganic and organic debris that will cause pH to fluctuate and nitrate levels to rise. This is something to be aware of and test as necessary.

Here is a little more information about rainwater and your koi pond.


A koi’s metabolism speeds up as the water temperature increases. So feedings can be more regular to keep up with a koi’s nutritional demand.

Automatic feeder above a koi pondSome enthusiasts will feed their koi twice a day while others will insist on as many as 4. Automatic feeders can be helpful for those koi keepers who aren’t around to keep up with the schedule.

There are numerous varieties and types of koi feed available (we use Blue Ridge Koi food), and many enthusiasts who swear by one or the other.

Check the purchase date on any koi food. Three months or older can mean that it has oxidized and won’t provide the necessary nutrition.

Types of foods can have an impact on water quality too. As some foodstuffs will pass through partially digested, they add more organic waste to be filtered out. Some enthusiasts advocate for a higher protein diet in the summer months for growth, while others advocate for lower protein but a higher frequency of feeding.

No matter the frequency or type of koi food, the general rule is not to feed anything more than the koi will eat in a 3-5 minute period.


Fish lice found on koi fishSadly, as your koi start to perk up, so do the parasites that plague them. Many of the parasites that lay dormant in the winter months come back to life in the warmer waters. They also reproduce more rapidly at higher temperatures.

Cleaning your pond in late fall and then again in spring can help to remove the media that some of the parasites call home.

Healthy fish are also more resistant to attack from parasites, so keeping the water parameters at as close to optimal as possible and not adding any koi stressors will reduce the likelihood of an infestation.

For a look at all of the potential parasites and common koi illnesses, read this article How to Recognize and Treat Common Koi Fish Illnesses.


As the weather becomes nicer, there will be an upswing in those critters that like to make an expensive snack out of your most valuable fish.

Don’t be like Ed Sheeran and continue to stock your pond so that the local otter population can eat like kings.

There are numerous precautions you can take to protect your Living Jewels from winged, two- and four-legged would be thieves.

Netting is one of the most effective measures against the winged critters. They tend to get tangled up in any lines that are crisscrossed over a pond. The smaller weaved netting also makes it difficult to access the koi underneath. Here is an article discussing koi pond netting.

This getpond.com article takes a look at 7 ways to keep predators out of your koi pond.

They mention:

● Caves

● Dogs

● Decoys

● Fishing line

● Motion sensors

● Plants

● Netting

Natural Disasters

Last but not least, there is always Mother Nature to contend with.

In recent years, there have been some incredibly powerful storms and devastating hurricanes that have ripped through South and South-eastern America. Many koi keepers lost much-loved koi and suffered extensive damage to their ponds and equipment.

As June rolls in, preparing for the worst is something that all koi keepers in vulnerable areas have to take into consideration, especially as hurricane season heats up towards the end of the summer.

Our recent article, Koi Ponds, Storms and Hurricanes, outlines some useful and necessary precautions for those that are most at risk.

Summer is meant to be enjoyed. Any new spring koi additions to your pond should be up to speed and you have extra hours of daylight to enjoy them, as well as your old favorites.

We hope that these points help to keep you around your pond and your koi, and enjoy the most time doing what you love most—watching your hale and hearty koi swim around a clear and clean pond.

5 responses

  1. I have a 2 acre pond with Koi in it. My Koi are not reproducing. I feel my other fish, Bass, are eating my babies. I notice my Koi often going along the edge slowly as if they are trying to swim up on my lawn. Why? Do I need to provide a place they can hide? If so what?

    1. richard :

      separate the koi, so they can do their thing. grassy spots are where they lay eggs and the male fertilizes them. they hopefully hatch in the safety of the long grass.

    1. the fishamn :

      don’t ever add Oscars to anything. they are bullies. they are dirty. they move everything around. omg. the pet store wont even take them back I suspect.

      “The Fishman” 30 years in aquarium maintenance, design, and coral propagation. loose the Oscars.,

  2. Fernando Urrutia :

    I’m having a hard time getting my koi to come up at feeding time, they hang out in the bottom of the pond, and will wait for the pellets to be pushed down by the stream of the water returning from the filter…

    I think that the problem it’s the heat (86-89 degrees on average) the pond has a shade and the thermometer on the pond shows around 82 degrees on average… What else can I do to keep the pond fresh?

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