[DAVES]

Originally Posted by JohnN

copper sulfate (pre-mixed or you can mix your own on the cheap) is great for mildew if you don't want to use bleach or outdoor bleach

Patio Moss Mold and Mildew Prevention

As said we use some pretty nasty stuff to clean. Copper sulfate is the stuff you put down your drain to kill tree roots growing into the drain line. It will kill a tree applied on the root system.