Zeolite is a product that the pond store will carry that will adsorb ammonia.
It looks like small white rocks and you place it in the water in a mesh bag.
Sorry about your koi.
You would be better off keeping the goldfish in the original container though.
It has had water and ammonia for about 2 weeks now. It takes 3 - 4 weeks for the bacteria to populate the tub surfaces, typically. By putting the fish in a new container, you will be starting the cycle all over again.