You can take your tank in and have it repaired, then have it pressure tested. The black you are referring to is probably inert, and harmless. Microbiological induced corrosion
(MIC) requires both diesel fuel
to proliferate, and creates acid which can etch through solid steel
additives, which you know about, are recommended, as is removing any accumulated water
in your tank. You are correct in that fittings below the fuel level are not advised, but a top-mounted permanent water removal
My fuel polishing unit is a Fill-rite 15 gpm fuel pump
, mounted in a plastic tote for containment, with a 15' hose for both pickup and discharge, running through a goldenrod particulate filter with a water settling basin.