Black Specks that wil not go away
I too have same problem and just ran across this site....I've read all the posts, and still appears that there is not a permanent solution. These little black dots/specks are extremely hard to remove from fiberglass tub & surround. I've used bleach (although hesitant too since have septic system) and other cleaning supplies but still will not remove unless I actually use my fingernail and "pick" / scrub them off.
These only appear in the bath in the basement. The house is 10 years old and the water heater is only a few years old now. No rubber gaskets anywhere in plumbing that I know of (except for O-ring to seal water filter and that is brand new).
I certainly think that it is a micro-organism of some sort as others have suggested, but am stymied as to why no cleaning solution, esp. bleach does not even "touch" these things.
Many of these are on the shelf of the surround where what does not hit so do not think it is coming from water heater or the water itself. Also, the hot water, as well as cold, appears crystal clear in a glass.
This almost looks like black over-spray from an aerosol paint can, if that helps.
Thank you for all your help.