I have a new tankless hot water heater installed at my house. When I run hot water in the tub, there are always small black particles that are easily visible (about this size of small seeds).. sometimes the water has a milky color to it that always clears.. but water sometimes looks a bit dirty.. Also there is an unusual organic scent that I can sometimes get a hint of which I have never smelled before in water. Cold water does not have these issues.

Any ideas?

What type of tankless is it? How where the connections made? Did you install it yourself, or did you hire a plumber to do the work? Let me know as much as you can and we will see what comes to mind.