Yes, you will need 1000 gallons for the first fish and 100 gallon for every koi after that. They get huge and produce 3 times the waste of a same-sized non-carp fish, like a cichlid. Koi release ammonia from their gills like goldfish and will out pollute you like it's a contest. Assume two and a half feet for an adult Koi. That's seven and a half feet of waste. Would you let a human go pooping in its water at less than 1000 gallons? It's not exactly equivalent, but worth realizing how much friggin' waste these guys exude all day. Less than 1000 gallons will make your life a very busy one.