Dude! I had a small 3 inch Koi and that thing took 3 years to reach another inch and a half!
You will be ok with a 75 gallon tank in your house with a Koi and maybe some goldfish, I kept mine with a Pleco and a goldfish and he did just fine!
I dont know where everyone is getting this bogus idea that they will reach 8 feet in length in 1 year
These fish take FOREVER to grow! So dont worry about tank space, I am not being negative about the situation, but if you can raise it to go past 3 years then you might wanna consider getting something a little bigger down the road. But its seriously not a big deal keeping Koi, they are just like goldfish but with whiskers! They eat and poop and swim around all day.
I guess..... Im not sure, I am just stating my observations in a 75 Gallon tank and my Koi took a while to grow, I wouldnt go as far as saying 3 feet in one year! I mean I would probably expect something like a RTC to grow 3 feet in one year HAHA