Koi get very large. If you put them in a pond for the summer, they are going to grow very quickly, too. My friend has a koi pond in her yard, and she has to keep heaters in it in the winter to keep it from freezing, because the fish are too big to possibly come in the house.

I don't think your fishies are going to be able to live in your tank for very long, and you are going to need to do very frequent water changes. You might want to start looking for an outdoor home for them now.