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Koi Care Page Two

Last Updated: 1/12/09

Setup and Water Preferences
Koi and Plants
Special Additions for a Koi Pond
Koi Herpes Virus

Setup and Water Preferences

Koi should be put in ponds of 500+ gallons. Experts say to have 1000+ gallons for the first koi and 100+ gallons for each additional koi. Koi prefer a moderate hardness, temperatures from 50 degrees F to the high 70's degrees F, and clean, well-oxygenated water. Koi can survive short amounts of time in temperatures as low as right above freezing and as high as 90 to 100 degrees F. Koi eat just about everything! That includes insects, fish food, and your prized water plants when it suits them. They usually do not seek out fish to eat but will eat eggs and fry of many species.


Males can develop breeding tubercules. During breeding, females are noticeably fatter. Outside breeding times, sexing is very difficult for people new to koi. I am not aware of any methods used aside from shape.


Left to their own in a large pond with both males and females, koi will spawn on soft plants in the shallows. The information on koi is similar to that of goldfish. In fact, goldfish and koi can produce offspring since they are closely related. The resulting mixed species fish are believed to be sterile.

Koi and Plants

As anyone who has tried to combine koi and plants will tell you, koi love to eat plants. I only had at most three butterfly koi at any time. One was about a foot long as of 8/31/98, and the other two were half as big. The large koi, Remmy, died during the spring of 1999. The two remaining koi I had then (10/99) had grown to Remmy's size as well. My koi love to root the pebbles out of the pots and generally make a mess. If you have potted plants with koi, be sure to cover the dirt with at least two inches of pea gravel. Even then, expect some dirt released into the pond. Some people suggest topping with larger gravel or lava rock that koi cannot move but I would think that would hold in less of the dirt.

When my pond was set up in May of 1997, there were some dozen two gallon pots with about 100 anacharis plants and 6-12 plants each of bacopa, cabomba, hornwort, moneywort, jungle val, foxtail, dwarf sagittaria, and red ludwegia (probably not a winter survivor). As of 8/30/98 when I checked the submerged pots, the koi (mostly Remmy who was a foot long but has since died) and goldfish had eaten all but about 3 bunches of anacharis, one large piece of hornwort, a few dwarf sagittaria, and some jungle val. Those must be the more koi resistant species to try. During the summer of 1999, both anacharis and hornwort fared very well in the pond.

My koi did not harm the lilies or marginals to any extent in 1998 but ate the water clover in 1997. A new water clover plant in 1998 took over a large part of the pond. While my koi eat all duckweed and most azolla that they can find, they do not eat salvinia (also called water velvet) to any extent because it grew to cover 100% of the pond surface by Fall 1998 (from just a few pieces in Spring). I was tossing out 20 pounds of it a week to keep an open spot in the pond. Salvinia would be a good choice for those wanting a floating plant that is koi resistant. Be sure to keep it in a ring or otherwise protected until it has multiplied to a hundred plants or so, each about an inch long.

There are two keys to keeping koi and plants together. One is to have more plants than your number of koi can eat. The other is to keep the koi well fed on vegetative rich food. The only way to insure koi will not eat something is to make it inaccessible by using blockades, nets, etc. Many pond catalogs sell special mesh cages for plants to protect them. I have yet to find any plant that koi or goldfish will never eat at all. Salvinia comes close. AquaMart sells many types of plant protectors for water lilies, submerged plants, and floating plants. I have ones for floating and submerged plants. The floating ones work well except when goldfish jump into it to spawn! Once the floaters fill the net up, I release them. The submerged plant protectors did not work well for me when I finally tried them in 2003. Maggie just pushed them down, untied them, and ate the plants right through the mesh! I give up on submerged plants!

Other methods to deter koi from eating or disturbing plants include the following. You can feed the koi duckweed raised in another pond (a tub pond is fine) to satiate their plant hunger. Plants can be raised in a separate pond connected to the koi pond by a waterfall. The top of plant pots can be covered in a mesh to keep koi from rooting out the rocks and dirt.

Special Additions for a Koi Pond

If the predominant purpose of your pond is to keep koi, then you should consider adding the following things to your pond.

1. A bottom drain to collect the excess debris off the bottom.
2. A skimmer to skim off leaves and debris.
3. An excellent filter system that is easy to maintain and can remove (and/or render less harmful) the copious excretions of koi.
4. Maybe a UV sterilizer (see here).
5. Shade provided by overhangs, suspended tarps, or mats of floating plants that the koi do not prefer to eat.
6. If you want plants that are safe from hungry koi, build a separate bog pond or vegetable filter, connected by waterfalls to the koi pond or otherwise blocked off from the koi.

Koi Herpes Virus

In 1999, koi owners and suppliers in the USA began experiencing koi die-offs. In most cases, the person obtained a new koi or brought a koi back from a show that infected the pond. Mortality was nearly 100% for koi and carp but seemed not to affect other species. Symptoms included those common to bacterial outbreaks, gill abnormalities, sometimes lesions, listlessness, etc. The fish did not improve with antibiotic treatment. Many koi specialists began working on the problem. On the East coast, a herpes virus seemed to be the culprit. It was isolated from infected koi. Secondary bacterial problems were commonly present. On the West coast, a virus was suspected but had not been isolated (as of 10/99). Koi can be infected directly or from being exposed to water containing infected fish. It is super contagious. As there is no known treatment for this problem, extra caution should be taken to quarantine all new koi or koi returning from a koi show. Quarantine tanks should be far away from other koi and no supplies should be shared between them. If the quarantined fish dies, all supplies and the tank/tub should be soaked in dilute bleach for a day. The koi groups suggest that you send in dying koi for testing to figure out this problem. For more information, visit any of the first three koi links below. The MAKC specifically has information and where you can send infected fish for analysis.

In 2000 and 2001, very few koi became ill and a new risky method of treatment was devised using near toxic doses of Chloramine-T. Using that protocol (I refer you to the MAKC as the treatment is risky, and I have not had occasion to use it), many fish were able to be saved. The virus was named viral gill disease and later koi herpes virus (KHV). The virus allows the invasion of various bacteria that may be resistant to antibiotics in koi often treated with antibiotics. The virus is contagious even after a few hours in water with a sick fish. All new koi should be quarantined as well as those returning from shows where they were exposed to other fish.

By 2008, it was known that using high temperatures kills off KHV. Here is one link:
Heating koi with koi herpes virus.

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