Had the black fungus in my tank a few years ago, looked like fur in the tank. I used the Biobor Diesel Biocide JF and had very good results. The only thing that will kill microbes in diesel fuel and keep them from spreading is a diesel fuel biocide. First why did it happen. Bet water is getting in the tank, check the deck
fill o-ring replace if cracked. The ultra-low sulfur diesel, is missing the bacteria-killing sulfur so that doesn't help.
The fix I used. I used the hole for the tank level sender unit to pick up trash/water from the bottom of you tank with a copper tube, do that first. Add biocide and fill the diesel tank. Recirculate the fuel 30-60 minutes or more for bigger tanks
. Let the diesel fuel settle. Do another trash pick up from tank bottom. Change the filters a little more often and you'll be good to go.
Keep using the maintenance
dose of the biocide and if fuel is sitting use stabilizers. Polishing the fuel does nothing to fix what caused the problem or in killing the microbes.
I did this and few years later had a guy polish my tank, said it was one of the cleanest tanks he had seen.