I have a lot of spider plants at home, which all used to have persistent scale insect infestations for many years. Rather than just dumping the plants, I had always tried to save them by plucking out the scales manually or spraying them with chemicals, but that never really worked. The scales had always came back in large numbers. But, sometime ago, about a year or so, the scales started to gradually declined in population until none remained.

I don't understand what's going on. I never changed the way I watered/fertilized/cleaned the plants or the tools used in doing so, and I never changed how I did things at my home. My guess is that something was added to the municipal water supply or chemicals are leaking from from the plastic bottles I water my plants with are harmful to the scales. Any thoughts, /an/?