Those of us with old diesel boats have seen a lot of that black sandy looking stuff. It is typically called asphalteens, and comes out of aged diesel fuel. The low sulfur diesel used an extract of asphalt, which would come out of solution over time. I would be surprised if asphalt is used in ULSD but it could be on the bottom of your tank. It can be dissolved by Startron tank cleaner, and likely by regular Startron used regularly. I have spooned it out of my sediment bowl on my boat. It is not microbial, fungal, or algae.

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'97 E 300 D