I use 1 teaspoon marine salt mix, 1 teaspoon baking soda, and 1 tablespoon epsom salt per 20 gallons. My KH is at 4 and pH at 7.2. Here is the link to the article which explains it all:
http://www.wetwebmedia.com/FWsubwebindex/fwh2oquality.htm
I'm sure people are tired of hearing me talk about this LOL but it really saved our pond, so I know that it works. It is also cheap and easy to measure. Just change the water over gradually to insure your fish don't get stressed.