Hi Nancy,
Being honest..
If you were going to keep one temporarily- meaning a month or so, maybe.
A couple of years.. NO.
Imagine having a VERY small cage which is just fine for a Chihuahua.
Then you get a Great Dane puppy and want to keep it in there for two years.
(just using that as an example but there are other reasons besides growth that koi need larger ponds.. better filtration, deeper depths, etc.)