However, they have been known to achieve sizes much greater, especially if they are raised in mud ponds and fed an optimum diet. According to Japanese sources, the average life span should be about 70 years. Some individual fish have been reported in excess of one hundred years or more, such as the legendary Hanako alleged to have lived for over 200 years. However, nowadays, if you follow Nishikigoi fancier's advice and acquire a general knowledge of keeping them, you won't let them die so often. Nishikigoi will grow rapidly and live long if they are raised in a proper way.
Proper filtration is essential to maintenance of a koi pond.
It is recommended that you have both biological and mechanical filtration, but you need at least the biological. The mechanical part, or prefilter, removes solids, and the bio-filter removes dissolved waste. Absence or poor maintenance of biological filtration generally leads to unfavorable results, ranging from proliferation of algae to death of fish. Ammonia, which fish produce as waste, is converted to nitrite by the beneficial bacteria in the bio-filter, and finally into nitrate which can be removed by plants and partial water changes. Depending on the type of filter system you have, cleaning can be done by back-flushing of the filter chamber or removing the filter media by hand.