Oh, I love koi fishes. My aunt used to have them in her aquarium, they are very beautiful and can change the look of a room if you have them as your aquarium fishes. My aunt has had her fishes for more than ten years, I heard they can live for up to forty years. I always help out with the care and maintenance of the fishes and the aquarium too. Here are some useful tips on how to take care of a Koi fish.

*The aquarium of koi fish should have adequate supply of water. It should have a ph of about 7 to 7.5. The fish thrives well in slightly alkaline water .

* The aquarium should be adequate light and contain sufficient nutrients in it. It is important to install a skimmer system or a pump to help with aeration and cleaning.

*There are feeds specially made for koi fishes. When purchasing a feed for you fish, be sure you are buying the one for the appropriate age. This is because younger koi fishes needs more protein in their diet than the older ones. You should add a little quantity at a time so that the water would not go bad.

http://fc06.deviantart.net/fs19/f/2007/291/8/6/Nishikigoi_Koi_Carp_Fish_6_by_FantasyStock.jpg

Koi is a very delicate fish and requires high levels of attention. They are also more intelligent, can be great friends Therefore special care is needed in rearing them.

The following basic things are to be kept in mind in rearing them ,which I read in a couple of article about caring of this wonderful fish . A summary of the same for your ref:
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The water quality: the Koi pond water should be kept clean by making use of Mechanical, Biological and UV filter media.pH level should be checked .

The type of pond: Concrete ponds should be avoided as they increase the pH level.

Feeding: Proper feeding habit should be maintained according to the seasonal changes---Twice during summer and once during winter.Shrimps are the best feeds.

Population: As koi fish is a colonizer it loves to be in groups.So we should make sure that there are atleast 2 to 5 in a pond.

Handling: It is a very delicate species so should be handled very carefully.Wearing a gloves is essential.Improper handling can damage their outer protective slime which gives them protection against fungus and bacteria.

Protection: building a net above the pond helps protect them from Fish-eating birds like herons if any around your area.

Diseases: In case of any dull,inactive are sick crap is detected immediately separate them .As they would affect the rest of the group.

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For further detailed ref do go through the source link mentioned below.

Not only are Koi easy to take care of, they are exceptionally hardy and long lived. They are able to withstand extreme temperatures, are undemanding in any season or climate and only ask for good water quality and nutrition in return. This is the only challenge to raising Koi as they like to eat a lot and produce a lot of water pollutants in turn. Koi eventually grow into carp so be certain that the container you plan for your Koi will be comfortable for them as they mature. If you plan to mix other fish, consider their size when full grown when planning space. The container can be indoor or outdoor and depending on which you choose will determine how it's cared for. Either way caring for Koi is a relatively easy proposition.