hi there
what size of tank and filter have you got?
you can keep koi in a tank but like tam said they will grow and fast
good filters and regular water tests and weekly dechlorinated water changes will be needed (make sure you only use the amount of dechlorinator for the size of your tank other wise over time a few weeks the water turns a funny colour and you will get a funny smell)
you will need to cycle your filters before adding the koi if you have a pond add a bit of the dirty filter wool from that filter to the new tank filter
watch the nitrates and ammonia levels they will build up fast in a tank and watch the ph as in a tank it can drop very quickly (if the water is very soft)
i have a 3 foot tank with a fluval 304 external filter and a fluval 4plus internal filter in this tank i have 2 koi (6in and 8 in)and 4 gold fish
if you are going to keep koi in a tank it will have to be at least 3foot gives them growing room and a good external filter
have a look here these are fairly cheap cheaper then what i payed and bigger
http://www.thelivingseas.co.uk/fluval-205-305-405range-388-c.asp
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http://mykoipaintings.piczo.com/?cr=7&rfm=y
http://pic7.piczo.com/myfishypics/