How many gallons of water do koi need, 20?

Try 250 and you'd be a lot closer

wow just for one
i learned something

...... and you should have two not one, or maybe one koi one goldfish. They get lonely and depressed which could cause health problems.

I have found that out already!

they need 250 gallons each, and you need more then one as they are very social fish and they may get lonely

Yep, 250 or more. I have mine in a 3,000 gallon pond Remember, they can get to be 3 feet long!

A general rule of thumb for koi that I've heard is 1,000 gallons minimum for the pond, allowing 300g minimum per koi fish in said pond.