Hard Water and Insecticidal Soap

Here's an interesting fact I came upon while researching homemade insecticidal soaps, but probably also applies to commercial insecticidal soaps also.

The minerals in hard water react with the soap and create soap scum instead of insecticide. Just like in your shower, if you have hard water. So always use pure water to mix with your insecticidal soap. I saw somewhere that Calgon™ or similar product could be used to soften the water. I would think distilled water would also work well.