That tank is not even remotely big enough, he won't be healthy for long in it. His growth will be stunted and he will eventually die, I'm so sorry to say. You need a pond with a minimum of 1000 gallons, as Koi
will reach 36" in length as Cody said. If your tank is below 40 degrees, (which I doubt, because your house would be quite cold, lol) you shouldn't feed your Koi
. Below 40 degrees and they cannot digest their food. I had a Koi
in my pond (before a heron got him, sadly) and I always stop feeding the fish in my pond once it gets cold out. They will do fine outside in the summers, though. But if you have him indoors in a tank with tropical fish that need warm temps, that won't be good long-term as he is a coldwater fish, same as Goldfish
. Is it all possible that you can dig a pond? I dug mine myself, although by the time you buy the liner/pump etc. it can cost a few hundred dollars and then some. I'm thinking of you've had horses though, perhaps you've got room for a pond.