Most of the people who have just begun to keep the koi want to have good koi, anyway. Nishikigoi has a lot of varieties, and there is no koi of the same pattern even in a certain variety. It is no wonder that they want this or that.

However, the problem is that you have to prepare a pond (an aquarium) if you want to keep it. The pond shouldn't be only a pool. A comfortable place to live in should be prepared for the koi. Then, the koi can grow up quickly and at the same time the keeping will interest you for a long time.

First of all, I recommend that you have a good pond. I don't mean that a good pond is not a large pond. The pond equipped with a water treatment device, which can provide the koi with proper water, is a good pond. The koi can't grow up healthily until such an environment has been prepared.

A filter tank is the place in charge of water treatment. Water polluted with fish faeces and leftover food is carried to the filter tank, the polluted water is purified in it, and then the purified water is sent back to the pond again. This is called "circulating filtration." The pond is a small isolated space. If the circulating filtration doesn't work well in this small space, it will not be long before you notice the koi ruin their health day by day.

It is said to be ideal that the size of a filter tank is to be one fifth or above of the volume of the water of a pond. If there is a limited space, you can install any other filter tank of other makers. It doesn't matter at all. Water purification in the filter tank is performed by microorganism called filtering bacteria. For that purpose, filter media to make the bacteria live in is necessary.

Once, rubble and sand in the rivers were often used as filter media, but today, artificial filter media becomes more popular among koi fanciers.

Rolling filter media, for example, which is a cylindrical brush, has a wide surface area to make it easy to be stuck by bacteria. It is also easy to clean. These merits put this filter media at the top in popularity.

The bacteria sticking to the filter media catch dirty water passing through the filter tank, and decompose the dirty water into clean one. So to speak, the water of the small pond is being purified as that of natural rivers is. Besides, oxygen should be thoroughly supplied. Besides, oxygen should be thoroughly supplied not only for the koi but bacteria. For that purpose, use an air pump to aerate the water.