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Preparing your Koi Pond for Summertime

Taking a look at the Summer Sizzle in Koi Ponds. Read our latest article in Aquarium Hobbyist Magazine

Throughout the summer months koi ponds demand the most attention. EVERYTHING BLOOMS IN SUMMER! This encompasses all of the elements in a pond including koi growth, bacteria/algae, aquatic plants, and those pesky parasites. Along with these areas to address, dealing with water evaporation and lower oxygen levels are additional seasonal concerns. Even something as simple as controlling the intrusion of fertilizer and lawn clippings require more of our attention through the summer months.

Let’s address each topic individually. 

 Koi Growth

Koi are at their growing peak through the summer months. Water temperatures are usually in the 70’s and with their ability to absorb food better, this is when feeding should be increased to 3 or 4 times a day.  Feeding koi on this schedule increases their fat content and prepares them for the cold weather season when their metabolism slows down and they eat much less. Since they are fed more often in the summer, koi will produce large amounts of waste. This places a greater demand on the filter which will require cleaning the media more often and more thoroughly.

Bacteria & Algae Problems

Bacteria and algae growth flourish with the hotter weather. Bacteria, in itself, is not a bad thing. In fact, the goal of any pond keeper should be to achieve a proper balance between the beneficial and non-beneficial bacteria. The ability to balance bacteria in a pond is the key to controlling algae. Koi ponds are a living environment and we should attempt to simulate their natural surroundings to the best of our ability. The closer we can provide this, the better the koi will thrive. Those unfamiliar with this concept may expect a pond to resemble swimming pool like conditions. In a pool, the goal is to kill all unwanted bacteria for our own protection.  But with koi ponds, achieving a natural balance is the key. I’ve found that clear water with a slight brown fuzz (like a pool table top) on the rocks is usually is a good sign of a balanced pond.  If the water is clear and there is a presence of filamentous (hair algae) on the rocks, it can be easily controlled with salt or other chemicals. Pond balance begins with understanding the role of the “nitrogen cycle”, which starts in the filter. This is where many of the biological processes takes place. The nitrogen cycle begins with broken down fish waste (bacteria) or any organic matter, which can turn into harmful ammonia. In the filter, pond water passes through the media where the beneficial bacteria live. The beneficial bacteria break down the ammonia into nitrites. Further along the cycle, the nitrites are broken down into harmless nitrates. As the water returns to the pond, the nitrates are absorbed by the plants, making the water safe for the fish and completing the cycle. This is why aquatic plants can play such a key role in achieving water balance.

Additionally, if a pond is located in an area of direct sunlight, it can also encourage algae growth. Installing some type of shade (at least 60%) over the pond usually helps. The use of a shade cloth or an umbrella placed strategically next to the pond can be effective and yet still be removed when the summer season ends.

Along with too much sunlight, a big problem with algae growth are excessive nutrients in the water, especially nitrogen and phosphorus. When fresh water is first added to a pond, bacteria levels are low. However, bacteria are ever-present and it will establish itself very quickly because the fresh water is full of life-giving oxygen. Often, this is why we see large blooms of planktonic algae (pea soup green) soon after filling a new pond. How do we address this problem? Manufacturers package and sell bacteria and recommended applying it when starting a new pond. But in my experience, with time and patience, a good filter and a properly sized UV light system will help the beneficial bacteria overtake the non-beneficial bacteria. In this way, the pond will eventually balance out. This can take up to a month or even longer. I do recommend using bacteria and enzymes products found in aquarium and koi stores when a pond has already been established to help maintain proper balance. However, because there are so many strains of bacteria, these types of products don’t always achieve the desired results at the start.  Most of the time, simply experimenting with different products for your own individual pond will usually provide the best answer.

Another algae problem arises when foreign substances are introduced into a pond. Remember to use caution around a pond whenever mowing lawns, adding fertilizer or spraying pesticides. The unintentional intrusion of clippings or fertilizer can actually add nutrients and feed the algae in a pond. Even more damaging can be the practice of carelessly spraying pesticides near a pond. Wind drifts can blow the chemical into the water and may kill many fish.

Remember, with the longer days and hotter weather, pond water can evaporate very quickly. Hobbyists often assume they have a pond leak somewhere, but systems with waterfalls or fountains can loose water very rapidly through evaporation because the “droplets” are exposed to the air which dries them out. Monitoring water levels and noting how much water is used daily is a good method of discovering if there is a leak or not. In many cases, if the pump can be shut off overnight, and the pond doesn’t lose water, then the waterfall area should be checked for leaks. If no leaks are detected and the weather has been hot, then the water loss may simply be due to evaporation.

Aquatic Plants

Aquatic plants thrive throughout the summer months and show off their brilliantly colored flowers all season long. Depending on where you live, certain types of aquatic plants will only grow in selected planting zones (i.e. water lilies will only grow in zones 4-11 in the United States). However, because they are constantly submerged in water, aquatic plants seem to flourish everywhere by simply allowing them to grow on their own.  In fact, they usually grow so well that it’s important to keep foliage trimmed and thinned out to prevent overgrowth. This opens the plant and allows sunlight in to encourage new growth. Plugging the soil with fertilizer tabs feeds the roots and encourages the emergence of more flowers, especially with tropical lilies. In addition to enhancing a pond aesthetically, aquatic plants provide shade for koi and help block sunlight. They also aid in completing the nitrogen cycle. It’s a good practice to secure the root system with a layer of 1” smooth river rock. Koi have been known to tear up aquatic plants, especially if they are spawning.


These are always a challenge to control throughout the spring and summer. Parasites are awakened when the water temperature reaches 68-70 degrees.  Koi are most vulnerable to them in the spring because even though the water warms up and the parasites become active, the fish’s immune system hasn’t been fully awakened yet. This is when parasites will most often attack the fish. Even in the summer, if a fish’s immune system has been compromised, they are vulnerable.  They will react by attempting to rub off the parasite against the pond bottom. This is called “scratching” or “flashing” and is usually a good sign that the fish has parasites. But to know which type of parasite it is, a microscope is needed to determine the exact pest. When a koi flashes, they can damage themselves from the intense scratching and open up a sore or lesion on their skin. As with humans, these can lead to infections which should be treated as soon as possible. There are many topical treatments including salves like Debride Ointment.  I have also found that dabbing the sore with the organic compound potassium permanganate with a Q-tip will help it heal. Potassium permanganate is also good for treating the entire pond for parasites. Caution is advised when using it because PP is an oxidizer and it can burn the fish. There are several other options for killing parasites which are effective, such as a salt treatment or using an over the counter products like Pro-form C.  Gill and body flukes are especially difficult to eliminate. They attack the gills or body and require medications such as Kusuri Fluke-M for control.  Fluke-M also dissolves well in Proform-C.  Using Proform-C together with the Fluke-M is the best preventive medicine for many common parasites. Other difficult pests to control are Anchor Worm and Fish Lice. Anchor Worm can be seen with the naked eye and can be removed with tweezers. Using Proform-LA will help to loosen the worm to remove it. It also helps when  controlling fish lice.

Parasites are often introduced into a pond when bringing in new fish to mix with the existing koi. It’s always a good idea to treat the pond with either salt or other preventative medications when adding koi to the existing population. Birds are also known to bring in parasites. If possible, make every effort to discourage them from entering the water.

 Summary (or Summer-y if you prefer)

As you can tell, the summer season is by far the most active of the year. Although many challenges arise, if addressed quickly and properly, it can be the most rewarding season of all. Pond life takes on a whole new meaning in summer. Everything’s in bloom and all one needs to do is imagine themself sitting next to their pond in a peaceful setting, sipping a cool lemonade, feeding their koi and drifting off to tranquil bliss.

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