I just bought 20 gallon glass made aquarium and 2 yellow koi carps fish about 2 inchs long with some other aquarium stuff. I filled water in it, decorate my new tank and drop them in. They are happy swiming and playing. I bought food for them also named Six-Seven hi-protein. I am feeding them twice a day. One of my friend told me that these fish can get big upto a foot long if i kept them in a big space. I don’t have any plan for buying big tank in future. These fish have to live in this tank for ever. So, i am wondering to know that how big they can be in my 20 gallon tank? Does any one know? Thanks Its been already more than a week that i bought them. And i don’t think shopkeeper will refund my fish. I haven’t see any kind of problem on fish. They are very happy once i go infront of them they come at to me and ask for food. I can’t return them. Actually my question was ”How big they can get upto in my 20 gallon tank”?