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Koi Deaths in Summer

First posted in the Koi Blog on May 18, 2005



As of this writing, the sweltering heat of Manila 2005 has already claimed the lives of three of my koi. The temperature is hovering around 38-39 deg C (100-102 deg F) these past two weeks. The funny thing is, we do get short spells of strong rains in between longer periods of scorching temperatures. Something weird is really happening to global climates nowadays.


Back to my dead koi – they are actually my three least favorite koi. This means that it isn’t true after all that only your favorite koi die. One is a 14-inch Kujaku, which had over the past year developed a cloud of gray and ugly-looking stains on its head, losing the clear, sparkling head that it had when it was just 5 inches long. The other was an emaciated Gin Rin Kohaku and the third was a Shiro Utsuri that had a defect (a yellow mark) on its cheek.



I don’t feel good losing these koi, but I do feel very relieved that, for a change, I didn’t lose my favorite koi this time. It would be harder for me to accept the losses if these involved any of my remaining Kohaku’s, or that full-bodied locally-bred Sanke that people often mistake for a Japanese-bred one.

Figure 1. Koi Deaths in Summer can be Avoided

High summer temperatures shouldn’t be a problem for
deep, well-circulated ponds. But my pond is not deep, having been built when I was still an over-excited, uninitiated newbie in koi keeping. Water problems in our area prevented me from making it deeper after I knew better, since the extra depth wouldn’t have mattered anyway if we didn’t have water to fill it up. But now I realize that maybe I should’ve gone that extra mile. Well, the dry season is almost over, so I guess making the pond deeper is less urgent now, and can wait until next year.

Hot weather can kill koi in a number of ways, the most common of which is
oxygen deprivation due to the reduced oxygen levels in the pond. Shading a pond and keeping the water well-circulated go a long way in keeping your koi cool, and alive. Experts say that at 90 deg F, only 7 ppm of dissolved oxygen can remain in fresh water, and koi start dying at 3 ppm of dissolved oxygen.  It is recommended therefore, to have an extra air pump or two to aid in oxygenating your pond during summer.


Low oxygen levels can also hamper your biological filtration, since good bacteria need oxygen.  As such, it would be a good idea to check your water parameters more frequently during hot summer days, just to ensure that the ammonia, nitrite, and nitrate levels are not getting out of control without you knowing it.


And have you noticed those unsightly clumps of dark green matter that cover the surface of your pond on hot days?  These are actually decaying organic matter in the pond that get precipitated at the surface of the pond.  These are solid wastes that need to be mechanically filtered out, and will never be eradicated by your biological filters.  Fortunately, they are also quite easy to get rid of, extremely irritating as they are. Just install a surface skimmer in your pond, which is basically an apparatus designed to suck the pond's surface water and filter out the wastes.



Lastly, parasites reproduce more rapidly during summer.  Watch out for fish lice - a single fish can contaminate the entire pond in several days.  Once that happens, it would be very difficult to eliminate them permanently without draining the pond.
Koi are cold-water fish, and having them in hot places like Manila certainly has detrimental effects on them. Still, koi keeping is becoming more and more popular in Southeast Asia. Well, there’s only one thing to blame for this – their elegant beauty that is so difficult to resist.


See also: Koi Care





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