You pond sounds too small for koi.
Current recommendations are starting with
1,000 gallons and then adding 100 gallons per
koi. So your pond should be 1500 gallons without
any fish in it at all. Add 20 gallons per 20 goldfish and that is another 400
gallons.
You should take the fish back to the dealer.

A fountain isn't going to do the best job filtering a pond with koi, they have
higher standards for their water quality.

It is the fault of many store owners who sell koi in that they don't inform you
what the fish really need. I've seen pet stores sell koi for aquariums!
ak!

Did you say other fish had been dying? You need to test the water. If you see
any trace of ammonia that means your fountain is not working as a filter.

good luck!


k30a