What is the best fish for me?

I need help to decide what fish is good for me. i have recently had a 5 year old gold fish that has died. It lived in one of my Farm water tank ( 20,000L) and it has no top so the fish get daylight. I didnt feed it normally fed off algae and little bugs in the tank. the tank stay cool and areated from the bore. i was going to get some koi fish but i dont know that much info on them. it would also be good if they could breed and adapt to new waters. be good if u could tell me what fish to get. sorry for the spelling