Hello, my question is... What kinds of fish could live in harmony with my sons 2 3-4inch comets without heater ofcourse? I was doing a little research and I saw that some species were mentioned, but I would love to get a koi... I think they are pretty Goldfish related fish. But I know that they get huge very fast and will quickly outgrow the 20 gallon I have. I really don't want to go more gallons at the moment, so i know a koi is out... but what other small fish could live in these conditions and would the two befriend as a "3rd wheel"?