Originally Posted by RichH
Looks like a very common diesel oil fungal contaminant - Cladosporium Resinae or "kerosene fungus" - a resin forming fungus that uses the diesel fuel as it nutrient source. That 'black' is most probably from a massive 'colony' growing on the walls of your tank and is periodically breaking loose from the walls as the dead fungal cells disintegrate and 'come apart'.
When you see such as this the only method to prevent further premature filter clogging is to clean the tank either by opening the inspections ports and SCRUB or add a 'tank cleaner' such as Starbrite enzymatic "Tank Cleaner", etc. ... or better, do both. If you dont, then expect to use a LOT of filters.
Such common fungal contaminations result in very 'soft and extrudable' particles that easily 'slime over' filter media which leads to premature blockage of the filter .... and until the blockage is 'total' the increasing pressure across the filter will cause this 'stuff' to extrude through the filters, and ultimately deposit in the hot sections of the exhaust system as a thick "coke".
Infected tank, constantly plugging filters, AND clogged, 'coked-up', exhaust system.
This is most probably an 'infection' - dont simply change the filters, clean the tank!
FWIW - why does 'non taxed/dyed' fuel from a marina ALWAYS cost much much much more than 'taxed' fuel from a truck-stop?
Unfortunately, none of the Startron products are bio-cides, capable of killing the bugs. They did market Star Brite Bio-Desiel, but it doesn't seem to be in the catalog anymore. Bio-bor, FPPF Kill-Em, and Raycor Bio-cide Diesel are proven products.
Rich is right though; a clean tank with NO water does not provide an environment where bugs can grow. Treat with a bio-cide, clean the tank, and then maintain with bio-cide + fuel treatment.