I have a 30 gallon tank for two common goldfish. One of my goldfish got sick and passed away a few months ago. My water quality is good and I waited 2 months to make sure my other fish stayed healthy before I got him a new mate. I went to the local fish store and got talked into a Shubunkin, which I found out when I got home and did my research is actually a koi. They had no idea what they were talking about. Now I know koi get very large, but how fast. I think I will need 7-10 months to build an outdoor pond, my koi is currently 3 inches. Will this be enough time? Also, is it okay to keep common goldfish and koi together? Do I need to get different food for my koi? Please give me any help or tips that you can.