Stinky screenwash

[Oriskany]

My screenwash stinks of an eggy, drainy type smell. I topped it up with a couple of pints of generic ready-mixed but the smell persisted. I have since syphoned off the fluid and noticed that the reservoir is covered in a black deposit. I have since re-filled the reservoir with a mix of plain water and disinfectant. Anyone else had this - I wonder whether the black stuff is fungus?