I have a pond with all types of fish in it and my goldfish do breed, what you need to put in the pond is some floating pond weed, the babies will stay in there and not many will be eaten by the Koi.

You will be suprised how many babies live as they are jet black and dont change colour for a year or so. I bet we counted at least 30 babies last year!

This is a pic of the type of stuff you need, you can buy it in any pond shop or garden centre.

It grows and spreads like mad, keep thinning it out before your fish breed, once they have done their stuff leave the weed alone until spring. We usually keep a 1/4 of our pond covered with the weed.

http://www.agric.wa.gov.au/ikmp/images/elodea.gif