As a casual aquarium owner and an apartment renter, I have kept 3 koi in a 75 gallon tank for about three years. They were 3" when I got them and are now about 6", 5" and 4" respectively. I enjoy their livliness and have not had any problems with their health. I bought my fish from a reputable pond store, so they're not expensive or fancy, but I really enjoy having them.

Here is a link I found useful for information about 'fish loading' for koi in tanks:
http://www.aquariumfish.com/aquariumfish/detail.aspx?aid=9604&cid=3784&search=
According to this author, you can calculate the total number of 'inches' of koi that are safe to keep per 100 gallons. The right filtration system will keep them healthy too.

In my opinion, if you can't have a pond, the right tank will do fine for koi. They do tend to splash, so be aware of your floors around the tank.

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